L._.
There's no excuse for putting a wallet full of money in someone elses care, under their van seats etc. It's not just not needed. She should have safeguarded her money. I know you feel bad. But this is a lesson learned and it's her lesson.
I also would not assume that the other boy is the ONLY child that could have taken it. Also, who's to say that someone didn't know how to break into the van without leaving evidence of it? I know that's the least doubtful scenario.