My son killed his hamster

A couple weeks ago my ex got our son a hamster. He's 6 and it was his first pet. My son wanted to bring the hamster back to my house but I don't do rodents so I told him the hamster stays with dad. Sunday after I picked him up from his dads, his dad said the hamster had gone missing and the cage was opened and he had been looking around the house and can't find it. He told me not to tell our son and he will just get a new hamster and say its the same one. I just said okay. Today I opened my son's backpack because he still had clothes in it from when he was visiting his dad and I needed to wash them and I about died. I saw a dead hamster in there and screamed bloody fucking murder! I asked my son why a hamster was in his backpack. What happened was he decided he wanted to take the hamster home anyway so he snuck it in his backpack and then forgot it was in there! I took him to get a happy meal at McDonald's after I picked him up from his dad's so I'm guessing thats where his mind went. I don't even know what to do with this child. I hate rodents, but that poor hamster. Lord Jesus help me.

He was very sad when he found out his hamster died and I had to give the death talk. It was not intentional. I understand it was an accident, but I do agree maybe he isn't ready for a pet.