The idea of shimmying into a pair of skinny jeans makes many women cringe. Now imagine squeezing them up over a diaper.

Ava Lane, a smiley two-year-old who lives in Deland, Fla., has four pairs of skinny jeans. She received the first pair as a gift, says mom Christina Lane, who thought they looked so cute she bought more.

Two-year-old Ava Lane shows off the fit of her skinny jeans. Molly Dempsey for The Wall Street Journal

"Babies and toddlers have big bellies, and skinny jeans are not for people with big bellies," says Ms. Lane, a 29-year-old museum marketing director. "But they still work."

Children's apparel, including skinny jeans for toddlers and kids, has been a rare bright spot in the beleaguered apparel industry during the recession and the recovery. Parents quickly cut back on purchasing clothing for themselves, but have continued to spend on their children.

Children's clothing sales are up 5.3% year-to-date, over the same time last year, according to MasterCard Advisors' SpendingPulse, a unit of MasterCard Inc. that tracks sales by cash, check and credit card. Total apparel sales are up just 1.4%.

"It's more fun," says Ms. Lane of shopping for her daughter than shopping for herself.

Most recently, Ms. Lane bought two pairs of skinny jeans from Gap Inc., the country's largest apparel retailer, which sells skinny jeans in sizes as small as zero to three months. The company says skinny styles now make up 40% of its jeans offering for infant and toddler girls and 70% for girls at the kids division.

WSJ's Ray Smith goes to New York's hipster central, Williamsburg, Brooklyn, to learn more about tight-fitting trousers.

Skinny jeans—slim through the thighs and knees, and even slimmer through the calves and ankles—are popping up in pint sizes at mall stores including American Eagle Outfitter Inc.'s 77Kids and Little77 divisions as well as J. Crew Group Inc.'s Crewcuts. Several denim players, including True Religion Apparel Inc. and Levi's from Levi Strauss & Co., also make skinny jeans for kids.

Skinny Jeans Gap designed its skinny jeans for kids and babies to closely mimic the shape and style of its adult jeans. Compare sizes 3-6 months, Girls' 6R, and Women's 6.

"People tend to put their kids first," says Mark Breitbard, executive vice president of GapKids and babyGap. "They'll pass on something for themselves to make sure their kids are still looked after." The company doesn't segment sales within its lines, but Mr. Breitbard says the kids' division "has maintained its brand health in a much more consistent manner" than the adult division.

One factor behind the company's skinny-jeans-for-kids strategy: The popularity of adult-styled kids' clothes after the debut of its kids and baby collaboration with Stella McCartney last November. It was originally intended as a one-time partnership, but the line was so popular that a second launched in March. Some of the pieces, such as a gray sweater dress, were miniature versions of Ms. McCartney's ready-to-wear line.

The mini-me mentality has trickled into Gap's other designs for kids and baby. On shelves now, shoppers will find motorcycle jackets and ankle boots. "People love trend take-downs," says Mr. Breitbard. "Fashion-right clothes are really adorable the smaller they get."

Translating a trend for the wee set isn't as easy as shrinking a pattern. Gap's new product piggybacks off its adult counterpart, 1969 denim. The overall goal was to make the kids' denim look more like the adult product. The similarities: Gap lined the inside of the waistband with blue oxford-shirt-type material, added a blue rivet near the right front pocket and the same detailing on the back pockets.

Christina Lane of Deland, Fla., zips daughter Ava into a pair of Gap skinny jeans, which retail for $34.50. Ava has four pairs of skinny jeans. Molly Dempsey for The Wall Street Journal

There are a few differences. For example, the "top block"—the top half of the pant covering the area from the waist to about the thighs—of the baby version needs to allow room for a diaper, so it takes up an astonishing half the pant. Other Gap jeans feature inseam snaps for easier diaper changing, but not the skinny styles.

Gap tests all its products, including babyGap's Mini Skinny and GapKids Super Skinny, in a special "fit room," where designers observe the product on the body, in action, says Jennifer Giangualano, head of design for GapKids and babyGap. The retailer also solicits feedback from parents who are there watching their kids. "It doesn't matter how cute it is," she says. "If they can't crawl around and move, it doesn't get out of the fit room."

Old Navy, a lower-priced division of Gap, also takes care when designing skinny sizes for little boys and girls, says Michelle DeMartini, senior vice president of Old Navy kids and baby design and merchandising. "We sent our teams out to playgrounds, not in a stalking way, but just to kind of watch the consumer," she says. For skinny jeans, "we talked for a long time about how much stretch, what should that feel like?" she says. "Adults might want to forgo comfort for fashion, but children will not."

Gap's skinny jean styles for babies have pocket details identical to its adult version, with criss-cross stitching and triangle-shaped seam reinforcement. Patrick Conlon/The Wall Street Journal

Gap based its pint-size skinny jeans on its 1969 adult label. Patrick Conlon/The Wall Street Journal

The smallest sizes—including zero to 3 months and 3 to 6 months—have elastic on the back of the waistband to make getting the jeans on and off easier. One feature the skinny jeans do not have: inseam snaps for diaper changing. Patrick Conlon/The Wall Street Journal

When applied to children, the "skinny" label sounds a tad odd. As comedian Stephen Colbert said during his Comedy Central show "The Colbert Report": "Thank you, the babyGap. I say, 'You are never too young to worry about the size of your fat'" bottom.

At babyGap and GapKids, prices for skinny jeans range from $19.50 to $39.50. Gap's marketing campaign for its kids and baby denim is "Born to Fit." The retailer features skinny jeans on its mannequins, which caught the eye of Sarah Coffey, a 32-year-old school counselor from Minneapolis. Ms. Coffey recently bought her 2-year-old daughter, Sebryna, her first pair of skinny jeans at babyGap. Before she committed to the purchase, she hauled Sebryna into a dressing room to check the fit.

"She's not skinny, so I wouldn't have necessarily thought to buy skinny jeans but I was inspired by the display," says Ms. Coffey. It featured a mannequin clad in gray skinny jeans, a striped tunic, plum-colored cardigan and silver ballet flats. Ms. Coffey bought the entire outfit. She does not wear skinny jeans herself.

But she says she might in the future. Children's clothing fills an important spot in the life cycle of some trends: After reaching young women, fads often swing downward to tots, and then back up to their parents. That's because many moms are more willing to be fashion-forward with their young children than with themselves, Ms. DeMartini says.

The children's department is "a safe place for the mom to start the trend on her child," says Ms. DeMartini. "She can vicariously live through that child." Ms. DeMartini dressed her kids—ages 2, 8 and 10—in skinny jeans long before she tried them. She says that eventually, her own wider-leg jeans started to look "weird," prompting her to experiment with skinnier styles.

And while many moms won't wear unflattering "mom jeans," they also won't squeeze themselves, or their children, into skinny jeans. "I have pretty normal-sized kids," says Nora Leibowitz, of Portland, who does not wear the style. "That means they're not skinny in any way." Ms. Leibowitz, a senior policy analyst for the state specializing in federal health reform, says comfort is her main goal in dressing her 5-year-old daughter and 1-year-old twins.

Gap isn't letting up. In recent weeks, jeggings, a hybrid of jeans and legging, hit GapKids stores. In the pipeline: Jeggings for babies.

Write to Elizabeth Holmes at elizabeth.holmes@wsj.com