The other day DD and I were at a fairly quiet soft play with only one another child and his mom. The kid (6.5) invited DD (3.5) to play. Everything was good at first, they are running around in circles, laughing, etc. Then suddenly he picks up pillows and hits DD with them quite hard (not sure if that was sort of a game for him too?). DD bursts into tears, I pick her up, cuddle her and she tells me a few times about how the boy hit her. The boy and his mom don't react although they both clearly see what's going on. I say to DD he 'did not do it on purpose' (although he clearly did) and we leave soon.
Should I have just said something to the boy's mum? I had a feeling that DD wanted me to talk to them but I was not sure what to say? What would you do???