Never allow children a direct line of access to anonymous masses, infinitely more so if they're girls. If they've a talent for performing to some degree and want to make pre-recorded skits and the like that they don't need to interface with the audience to produce, fine, go for it, but no audience-to-performer contact without heavily filtering it for fucking scum first.
Once they're legal majority it's whatever, they're legal adults that can start learning to sort that shit themselves. Still though, how entertaining would an actual child vtuber really be in live streaming? If they have a strong commentary game then fine enough but there's no shot a 9 year old has anything going on in their life that is worth talking about that I at over 4x that age will be interested to hear. They barely have started developing their personality and will have hardly any life experience, compare against, say, Randon who I could listen ramble about a fucking ball of sticky cling film melting in his car and be enthralled.