Legally, an 8 year old can not "own" anything.
So the money was not hers (again, from a legal standpoint)...the money belonged to her parents, held "in trust" for her.
That said, if the father took the money and used for his own purposes? Her mom can sue the father to recover that money. And this is something that can be resolved in small claims court.
Small claims is a true "peoples court". The formal rules of evidence and procedure do not apply. So you can represent yourself (no need to hire a lawyer).
The process is simple enough. You head to the county court (the county where the he lives) and talk to the clerk of the court. They can get you the forms needed to start. You file the claim, serve a copy on the defendant and wait for your day in court.
In court, be prepared to tell the judge what happened, and bring in copies of any evidence that supports your case.
Please let me know if you have more questions. I am happy to help if I can. Otherwise, please rate the answer so I may get credit for my work.