Only someone very sick in the head will steal from a child. Unfortunately there are some opportunists waiting to pounce when people let their guard down.
You can tell your little boy about my experience and may be it will help ease his pain. I went to boarding school when I was 5 years old and in my first week there, some kid (or adult) stole my beloved doll. There were about 2000 kids in this boarding school and it had four different buildings so the school was quite big for a five year old and I just didn't know who to tell about my pain. I tried to tell one of the teachers who simply ignored me by saying "You must have misplaced it". It was a depressing start at this new place and it took me a while to come to terms that I won't be able to get it back and I have to let go. I think when someone steals from us we feel violated and that just makes us angry and at the same time a sense of helplessness sinks in.
I think you can just listen to how your son feels about the incident and hug him as this is just one of those bitter experiences of life. As much as we hate it, life teaches us these things to help us avoid greater loss in future (at least that's what I've been told). See if there is anything positive he can take out of this experience, he has a loving mum by his side to comfort him in times of trouble.
Thank you for your perspective, doll. A great way to look at things. 🦋