Put posters signaling you found something (do not mention it’s cash) and would like to return it if the person can say what it is on the phone.

If someone hasn’t claimed it in 2 weeks, I’d give it to the kid (or we’ll probably her parents).

I’d also ask the kid were exactly they got it. If it was in a tree hollow or something similar, it might have been hidden and meant to pay for a drug deal or something. You never know, I just wouldn’t want that person contacting me.