IANAL, but a plane is not a lawless zone. If someone assaulted you physically, they would be detained upon arrival. I don't see why vandalism and damage to property would be acceptable. I'd have asked for a police meet and greet upon arrival.
This behavior sounds very strange, though. Did the kid seem normal? It sounds more like the actions of someone with a developmental disability.
Either way, the mother should be held accountable, if not the kid. I wouldn't have left the airport w/o a police report filed. What if they'd stolen stuff and not returned it? What if they'd engaged in some identity theft?
The airline knows exactly who they are. It seems to me strange that this would just be let go.