To accept kids under 13, the site needs to be compliant with COPPA. That implies at least things like:
a formalized Privacy Policy to be made available on any website exposing content describing or created by any child under 13.
legally-verifiable proof of parental consent to allow the child to sign up - signed statement, approved credit card transaction, recorded phone call, or digital signature.
parent-controlled overrides to view all the child's data, cancel communications from the site, control disclosure of information, deletion / revocation of the child's account
To accept kids under 13, the site needs to be compliant with COPPA. That implies at least things like: