I would also change the locks. I would tell your ex that you expect him to pay for the cost seeing as he is the one who "lost" the keys. I would be worried that you heard about this from your kids and not your ex. Your ex is the one who should have told you what happened.

I would also think about giving your kids the keys. I had a key when I was in early elementary school. I wore it on yarn around my neck so it wouldn't get lost.