Make it not a pay to win game. People are being mean because they think they are better at everything because they have a membership. Scamming is usually also because of memberships.

Teach your child about what to stay away from, use restricted chat (don't recommend bubble because then they basically can't talk) Tell your child to not give they're password to anyone else and teach them about scams, tell them to report and block jammers who make them uncomfortable or sad, and to not hesitate to tell you about it. Make sure they have a strong password. Tell them to ignore bullies and block them.