As you know, all homeowners policies include personal liability coverage, say $300K. Correct me if I'm wrong, but I thought parents can be held liable for the actions of their minor children.
I did a bit of googling, and came up with the paper at the link below. According to its author, the case law is mixed when it comes to arson by children of policy holders. It seems to matter what the intention was of the arsonists. For example, if they started a fire not intending it to destroy a building, then the damage might be covered. On the other hand, if they set out to burn down the structure, that would be a non-covered "intentional" act.