I think this is character development -- the reaction will depend upon the child's personality. First of all, when the child is unjustly accused of stealing, I think she would hotly deny it. But, she may be so shocked by the accusation from a powerful person that she is almost tounge-tied. Or she could just burst out crying, for the same reason. (Or, arguably, wet her pants.) What do you WANT to happen??
Second, when she finds the money...even intelligent children are usually not very subtle or manipulative (unless they have been manipulated all their lives.) So she might just turn the money in, and then be surprised when no one believes that she just found it. Or, she could figure she's already been "found guilty" so she may as well keep the money... and give it to her parents. Again, whatever you want to happen, can happen.