YouTube has been accused of violating child protection laws in the US, by a collection of 23 consumer, child safety and privacy advocacy groups.
The coalition has filed a complaint with the Federal Trade Commission (FTC) alleging that YouTube collects data from children aged under 13.
It said YouTube was "skirting the law and profiting off of children without parents' knowledge or consent".
In a statement, Google said: "YouTube is not for children".
Why has the group complained?
The group, which includes the Campaign for Commercial-Free Childhood (CCFC), argues that YouTube does not meet the requirements of the Children's Online Privacy Protection Act.
Websites run for children must inform parents if they collect personal data, and must seek parental permission before tracking data about children.
The group alleges that YouTube collects location data and the browsing habits of its users - even if they are children - and uses it to target advertising.
"YouTube is one of the most popular kids' websites in the world, and they know it," the CCFC said.
"Many of YouTube's most successful channels feature nursery rhyme videos, cartoons, toy ads and other content designed to capture young children's attention. YouTube provides how-to guides for creators making videos for kids.
"Google knows kids are there, and they are not taking steps to protect their privacy."
How has YouTube responded?
Google said the YouTube website is designed for people aged 13 and over, and people should be at least 13 years old to register an account.
The company said it had not received the group's complaint, but said protecting children and families was a "top priority".
"We will read the complaint thoroughly and evaluate if there are things we can do to improve," it said in a statement.
Google said its advertiser tools did not include the option to target adverts at under-13s.
It also said it offered the YouTube Kids app "specifically designed for children".
However, Google has been criticised for using algorithms to curate the app's content. Inappropriate videos have repeatedly appeared on YouTube Kids.
On Saturday, BuzzFeed reported that YouTube was planning to release a human-curated version of the app. However, the company has not confirmed this.