UPDATE: so the found amount was $160. we ended up making a vague poster saying "valuable found in area. Send description to email below". We waited a few days and got an email from a guy saying they lost $40 in the area. We waited a few more days and then the kids offered to give $40 from their find to the random guy as a good deed and we agreed they could keep the rest.
UPDATE TO UPDATE: I coordinated to meet with the guy (I didn't want the kids meeting with a random guy), but the guy didn't show up, so I gave the kids their $40 too
I wonder what happened to that guy.