Tried to teach my son a valuable lesson, it was ruined by another kid & his awful mother

This happened a few days ago and I'm still really pissed off about it. I have a 5 year old little boy who is very sweet and loving and smart, but has some behavioral issues. One thing we've had trouble with lately, is getting him to empathize with other people when something doesn't go the way he wants it to.I decided to take him to the state museum to explore and have a nice little outing. We ended up in the children's room, which is a super fun area with a bunch of different sections where kids can play dress up, build stuff, check out bugs and rocks and just all kinds of cool stuff. As we were leaving the bug area, my son noticed an Iron Man action figure laying on the floor and picked it up, asking if he could keep it. It was about the size of a barbie doll, maybe a bit bigger, and probably wasn't cheap. I thought this would be a good chance to get my son to try and think of someone else and see how good it can feel to do the right thing, so I told him we were going to bring it to the desk and see if we could find the little boy who it belonged to because he was probably really sad. My son was a little disappointed, but he said ok and didn't argue or anything.We walked up and he handed it to the guy, who said he actually saw the little boy it belonged to and they had just left the kids area a few minutes ago but he would keep it safe for him when he came back. My son was happy & we kept playing.The museum closed about 20 minutes later, and as we were walking out, we noticed Iron man was still as the desk. I watched as a little boy looked around for the worker, snuck behind the desk, and grabbed the toy. I didn't want to automatically assume, so I smiles and nicely asked him if it was his. He laughed in a mean way and said "it is now!" His mom was standing right there while it all happened and didn't say a word. I said, "well hunny that belongs to someone else and they're probably going to be pretty sad if they don't get their toy back. Wouldn't you be sad?" I'm thinking his mom is gonna jump in and back me up at anytime, but instead, she gets shitty with me and says, "why don't you mind your business? The museum is closing and he found it."And I admit I got kind of pissed, and told her that actually no, my son found it and I was trying to teach him how to be a good person but apparently that was a waste of time.I'm so aggravated about it because of course, he wanted to know why the other boy got to keep it and not him. He didn't learn any lesson, except stealing gets you a toy and doing the right thing gets you nothing. via /r/Parenting http://bit.ly/2ZLPtN5