As the Federal Trade Commission mulls changes to a law protecting kids' privacy, doctors and consumer advocates are urging regulators to investigate companies' "opaque" digital marketing practices targeting children.

In a letter to the FTC, the American Academy of Pediatrics, Center for Digital Democracy and 29 other groups asked the federal agency to review the information digital advertisers collect from kids, including data from cross-device tracking, machine learning, virtual reality and real-time measurement.

"As children become more digitally connected, it becomes even more important for parents, pediatricians and others who care for young children to understand how digital media impacts their health and development," Kyle Yasuda, president of the American Academy of Pediatrics, said in a statement.

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The consumer groups are calling for an investigation just as the FTC is conducting its 10-year review of the Children's Online Privacy Protection Act, the federal law that bars social media companies from collecting information on children under the age 13 without parental consent.

The agency last updated COPPA in 2013, but decided on an earlier review this year to consider changes around education technology, voice-enabled connected devices like Alexa and general audience platforms like YouTube that host kids' content.

"It's well-documented that compliance with COPPA is uneven among apps, connected toys and online services," Katie McInnis, policy counsel at Consumer Reports, said in a statement.

The letter also reflects growing concerns around the use of ads targeting children, such as new "playable" spots or those with a game component.

"What is the likely impact of these new methods on a children's psychosocial development, and what is the impact on family and social interactions?" the groups wrote.

COPPA violations have already surfaced several times this year. In February, the FTC fined Musical.ly, the popular social media app now known as TikTok, $5.7 million for collecting children's personal information without parental consent. In September, the federal agency also slapped Google with a record $170 million fine for violating children's privacy on YouTube.