After a research report last week found that YouTube’s advertising practices had the potential to undercut the privacy of children watching children’s videos, the company said it limited the collection of viewer data and did not serve targeted ads on such videos.

These types of personalized ads, which use data to tailor marketing to users’ online activities and interests, can be effective for finding the right consumers. Under a federal privacy law, however, children’s online services must obtain parental consent before collecting personal information from users under 13 to target them with ads — a commitment YouTube extended to anyone watching a children’s video.

Now Fairplay, a prominent children’s group, is challenging the company’s privacy statements. The group said it had used advertising placement tools from YouTube’s parent company, Google, to run a $10 ad campaign this month targeted at different groups of adults, exclusively on children’s video channels.

The ads were shown to users in consumer segments selected by the children’s group — including motorcycle enthusiasts, high-end computer aficionados and avid investors — on popular channels including “Cocomelon Nursery Rhymes,” “Talking Tom” and “Like Nastya,” according to a placement report Fairplay received from Google. In total, the group’s ads were placed 1,446 times on YouTube children’s video channels.