Beyond the Kids category rules, any app that's plausibly used by children needs to avoid collecting personal information without a real parental-consent mechanism, and can't include behavioral advertising targeted at that age group.
Common in general-audience apps that turn out to have a large child user base but weren't built with the stricter data-collection consent flow that requires.
"Your app collects personal information from children without obtaining verifiable parental consent, per guideline 5.1.4."