Google’s 6-week-old foray into video for preschoolers provides a seductive and seamless mix of kid-friendly fun and advertising, child advocates charged Tuesday as they called for a federal investigation of YouTube Kids.

In a 12-page letter, 11 organizations that advocate for children and consumers asked the Federal Trade Commission to probe the free Web application.

John M. Simpson, privacy project director at Washington-based Consumer Watchdog, said YouTube Kids “is full of content that is advertising masquerading as some sort of story … What’s happening now is unfair and deceptive, and what needs to be brought to bear are the same regulations that apply to children’s advertising in broadcast television and cable television.”

Google’s media team wrote that it rejects “the suggestion that no free, ad-supported experience for kids will ever be acceptable. We disagree and think that great content shouldn’t be reserved for only those families who can afford it.”

The clash enlivened the debate in the child development world over the Internet’s role in preschoolers’ lives.

YouTube Kids “is so attractive. That could mean that kids could get buried in it for a long time. That may be inappropriate for a young child,” said Christina Groark, co-director of the University of Pittsburgh Office of Child Development. “Children under 5 shouldn’t be left alone with computers and the YouTube world.”

Google, which owns YouTube, got input from child advocacy groups prior to the February rollout of the kid video channel. It promoted YouTube Kids as a safe online neighborhood for browsers ages 5 and under.

YouTube Kids users can’t click through videos to related websites or to buy things. Parents can turn off the search feature, and can set a timer so their child’s screen time is limited.

“I’m personally shocked that anyone would let a small child browse YouTube content unassisted, but I guess that’s what the app is trying to be a stand-in for,” wrote Harley Baldwin, vice president of design for Station Square-based Schell Games, which makes computer games for kids, when asked to comment.

A Canadian company, Kidoodle TV, offers parent-controlled, ad-free video browsing for $5 per month. “We don’t view it as being fair [to show ads], when you’ve got young kids that are impressionable, and they’re being targeted by an industry that has 80-plus years of experience in television,” said that company’s president Mike Lowe.

While children’s television offerings may have toy tie-ins, federal rules ban product placements, require clear separation between ads and shows, and limit the amount of advertising per hour.

When advocates test-surfed YouTube Kids, they saw what Mr. Simpson called “an insidious way that commercial content is being forced on young children who just don’t understand the difference yet” between stories and ads.

A McDonald’s channel on YouTube Kids, according to the groups, includes lengthy videos of Disney-themed dolls enjoying drinks decorated with the Golden Arches, and news report-style clips on subjects like “What are McDonald’s Chicken McNuggets made of?”

The groups zoomed in on “unboxing videos” available on YouTube Kids, in which children open, play with and review new toys. Some, according to the groups, are sponsored by undisclosed companies. Separate eight-minute videos showed, for instance, hands opening and building LEGO sets “while happy sounding music plays,” according to the groups, and kids assailing a pizza while a mother exclaims, “Domino’s Pizza! Cheese Pizza! HobbyKids love pizza. Do you love pizza?”

The groups believe that Google is tracking kids’ preferences and sending ads based on their clicks.

Mr. Simpson said his staff searched for McDonald’s on the app. “All of a sudden it just kept recommending more McDonald videos,” he said. “Somehow, somebody’s tracking something if you start getting recommendations based on what kinds of things you viewed.”

Ms. Groark said that she is worried about Internet companies basing advertising on a child’s video choices. “To me, personally, it seems wrong to send that kind of advertisement to the child themselves,” as opposed to directing it to the parent, she said.

Google maintains that because no sign-in is required, the app doesn’t collect personal information about the user and complies with the Children’s Online Privacy Protection Act. That act bars companies from secretly tracking the browsing of users known to be under 13 years of age.

Petitioning the FTC are the Georgetown University Law Center Institute for Public Representation, the Campaign for Commercial-Free Childhood, the Center for Digital Democracy, the Center for Science in the Public Interest, Children Now, Public Citizen, Corporate Accountability International, Consumer Watchdog, the Consumer Federation of America, the American Academy of Child Adolescent Psychiatry and Consumers Union.

An FTC spokesman said the agency “has received the [groups’] letter and will review the concerns.”

In September, Google agreed to pay $19 million to resolve FTC allegations that it allowed children to make in-app purchases without their parents’ consent. In 2011, Google signed a 20-year consent agreement with the FTC barring it from what the agency called “future privacy misrepresentations.”