Twenty minutes later, other parents showed up and a line began to form down Atlantic Avenue in Brooklyn. One father kept a list so that anyone searching for a thawing hot coffee could do so without losing a place in the line. He abandoned that project as more and more people trickled in and the end of the line was no longer visible from the front.

Some parents stood, shimmying and hopping to keep warm. A few line veterans brought chairs and buried themselves under blankets. It was too dark to read so they chatted about things like schools or children, and they poked fun at one another for being there. Every few minutes, someone would check his watch and express the hope that Carmelo the Science Fellow would open his doors early for his annual summer camp registration.

If waiting in line in the predawn of a January morning for science camp registration sounds crazy, you do not have a New York City child born after 2004. For those children and their parents, especially in the neighborhoods of brownstone Brooklyn, Lower Manhattan and the Upper West Side, not getting into activities, classes, sports teams — and even local schools — has become a way of life. If every generation must have its own designation, call theirs Generation Waiting List.

Looking for a spot in a public prekindergarten program in Lower Manhattan? Put your name on the waiting list.

Ballet for 3-year-olds at the Mark Morris Dance Group in Fort Greene, Brooklyn? The class is full, but they do have a waiting list.

Parks Department swim classes? Full. But maybe you can get on a waiting list.

At first blush, the waiting lists are a little surprising, given that in New York City there were 7 percent fewer children 9 and younger in 2011 than there were in 2000, according to census findings. Indeed, every borough has seen a decrease in children in that age range.

But the distribution of children is highly uneven, and some neighborhoods, especially those deemed "family friendly," have seen population explosions that outpace the general population growth, according to an analysis of census data by Andrew A. Beveridge, a sociologist at Queens College.

In Battery Park City-Lower Manhattan, the 9 and younger population has grown by 129 percent over the last decade; uptown, the Lincoln Square neighborhood has seen a 56 percent growth.

In Brooklyn, Park Slope had a 2 percent increase and its more affordable neighbor, Windsor Terrace, grew by 11 percent. The mostly Hasidic Borough Park neighborhood saw a 25 percent increase.

"The people having kids these days, they are a lot more well-off," Professor Beveridge said, "so those parents are much more likely to have kids who are clients" — of summer camps, music schools and the like.

On Laura Congleton's first day of motherhood, she waited for a delivery room at what is now NYU Langone Medical Center. So many mothers were giving birth, they were kept in waiting areas until delivery rooms became available, she recalled. Five years later, she is still waiting — this time for kindergarten. In her Brooklyn Heights neighborhood and adjoining Cobble Hill, the number of children under age 9 has jumped 31 percent since 2000, leaving her son on the waiting list at local private schools.

"There are too many kids," Ms. Congleton said, and too few spaces for them. "I just wish there was more room."

In some cases, the growth in the numbers of children clamoring to get into the same activities outpaces even the demographic change. Nationally, enrollment in the American Youth Soccer Organization has dropped 8 percent over the last five years. But in the Brooklyn region that encompasses Park Slope, Ditmas Park and Kensington, and draws players from Brooklyn Heights and Carroll Gardens as well, the number of children under 8 who play has jumped 43 percent in the past seven years, to 600 children from 420, said Ainslie Binder, the region's director.

Except for Brooklyn Heights and Windsor Terrace, the under-9 population from many of the neighborhoods feeding the league grew by only a few percentage points. Registration for the fall season is open for a month, from June 1 to July 1, but "it's like anything in the city, if you don't jump on it, you won't get in," said Elizabeth Kenney, a Brooklyn mother whose 9-year-old son was relegated to the waiting list last year and had to sit out the season.

Maria Lord also found that lesson out the hard way. Last year, she applied in July but found that every team was filled. Her son never made it off the waiting list. She does not plan to repeat her mistake. "We are definitely going to try to get in this year," she said. "There's this need for affordable team sports, but there aren't enough leagues and the demand is so great."

Besides population density, social mobility drives the waiting lists in certain neighborhoods, said Tamara Mose Brown, a sociologist who teaches at Brooklyn College. It is no longer a given that people who came to the city from the suburbs as single adults will return to them when they have families, she said. Those who stay tend to settle in neighborhoods where people are similarly educated and share like values. They want their children to experience the diversity and spontaneity of the city, but they also want to control the youngsters' exposure to those things by keeping them within a neighborhood bubble.

The more people bump into one another, the more ingrained a family becomes into a community and the more information will be exchanged about classes, or public schools. Those connections create cultural capital that helps families socially advance in their worlds. But it also puts them into competition for the limited number of slots for the most highly sought-after activities.

"All of this capital creates a certain type of anxiety among parents which, in turn, creates the 'waitlisted' child," Ms. Mose Brown said.

Brett Sonnenschein of Carroll Gardens feels that anxiety. He was one of those standing in line to register for Carmelo's science camp. Six years ago, when his daughter was applying for a private pre-K, he and his wife did not rush down on the first day applications were available for their first-choice school. Their daughter ended up on a waiting list there. Ever since, Mr. Sonnenschein been conscientious about tapping into playground gossip on what programs fill up first — and what steps to take to avoid ending up on the waiting list. "We've had many years of paranoia about this kind of thing," he said with a laugh.

Havona Madama's fear of waiting lists led her to start a database to track her 5-year-old daughter's favorite classes and their registration deadlines. Two years ago, she decided to leave her law practice to turn her research into KidKlass.com, a hub of information for brownstone Brooklyn about classes, camps and all-important registration dates. The site is still being developed, but she counts 50 to 100 visitors a day who peruse the listings. Still to come, she said, is an "alert" system to let parents know what deadlines they are about to miss.

One of her more popular offerings is Camp Panic — a fair in May at which parents who have not made plans for their children can see which summer camps still have room. "I can't stand to be on a wait list," said Ms. Madama, who lives in Carroll Gardens. "I don't like to waste my time waiting to see what the schedule is going to be."

Technology has fueled the phenomenon. In 2012, the city moved to online registration for its free summer swim classes at its outdoor pools. The number of applicants jumped to 34,134, from 20,393 in 2011, when officials began to introduce the online application. (That year, four pools still required on-site, in-person registration. Most people got in.) Last summer, only 24,532 applications got spots.

Often, the activities that fill up fastest are the ones that are most affordable and most accessible, like the swim classes. At the New York Public Library in Bay Ridge, Brooklyn, 25 children can be accommodated at the free story-time sessions. Parents and other caregivers routinely show up when the library opens at 10 a.m. to get a ticket for the 10:30 a.m. story times on Mondays and Wednesdays. On a recent Wednesday, tickets were snatched up within five minutes.

For children, waiting on a list for soccer or missing story time might not be a tragedy, but for parents, winding up on a list can mean having to put life on pause. In the Brooklyn line for science camp, the parents talked about how getting a spot could determine whether they could go to work on particular days, or whether they would have to spend extra money on a baby sitter.

At Public School 139 in Brooklyn's Flatbush neighborhood, parents showed up at 5 a.m. on the second day of school to sign their children up for the city-financed after-school program run by a nonprofit community organization known as CAMBA. Because of budget cuts, slots in the program were cut back: This year, about 52,000 children are being served citywide, compared with 85,000 in 2009.

Sandra, a single mother, could not afford to line up then, because she did not have child care. As a result, her daughter, who had been in the program before, could not get in this year. "I wanted to cry my eyeballs out," said Sandra, who asked to be identified only by her first name to protect her job as a nanny. Instead, her daughter goes to a public library every afternoon, staying until 6 p.m., when it closes. Over the summer, Sandra reviewed with her daughter how to walk down the street, turn the corner and find the library. "I had to teach her to be responsible faster than normal. I had to have her grow up," she said. "She's only 8!"

Parents in the Riverdale section of the Bronx or in northern Manhattan have long taken their children farther south into Manhattan or ventured north into Westchester for activities because their own neighborhoods had few. That is starting to change as offerings increase — but there are waiting lists. too. Leonisa Ardizzone opened Storefront Science in Washington Heights last year, and almost immediately had a waiting list for the first two weeks of her summer camp. She thinks this year will be similar.

Nearby in Inwood, Bread and Yoga has an after-school program offering classes like children's capoeira and art that fill up within the first week of registration. If you want in, you have to act fast, said Jo Flattery, a Washington Heights resident.

"It's just a fact of living in the city," Ms. Flattery said. She has learned not to discuss classes with her children until it is certain they will get in. She also follows a strategy that may add to the waiting lists. "You fill up every class you can, and you drop if you don't need it. Everyone overschedules — it's the only route to choice," she said.

The first year Carmelo the Science Fellow offered his summer camp, in 2006, there were 20 students a week. Now there are 120, and by 10 a.m. on the day of registration, most slots were filled. When the doors opened at 7 a.m. (an hour ahead of schedule), parents quietly filed past terrariums with heat lamps keeping snakes and lizards cozy.

Their wait ended at a desk where Carmelo Piazza and his wife, Karen, greeted many of them personally, with handshakes and kisses and questions about how the children had been.

Parents handed over a deposit and a form enrolling their children in camp and, just like that, on the last Sunday in January, their children's summers, and perhaps their own, fell into place.