Why is everyone attacking the parents?!
Not so much attacking as a bit stunned that anyone could think signing a 9 year old up to TikTok is a good idea. I mean, seriously 
I bet there's been some parenting decisions that you've made that others won't have agreed with
I don't doubt that, we all see things differently; this is pretty basic though.
Judging isn't helping.
I think it sometimes it does. Maybe someone will be reading this and take the comments onboard, this being able to protect their child. The issue here for me isn't the money so much as what the 9 year old has been exposed to.
I'm with you OP I didn't realise you could send money on the app and therefore it wouldn't have occurred to me to look for it.
Would it occur to you not to sign your 9 year old up to TikTok in the first place? Because most parents should know that.
All you can do is contact the bank/ the influences, maybe tweet them so it's public and ask them in good faith to refund the money asshe's nine.
I do agree OP should at least try to get money back. I don't know how successful she will be.