My two youngest DCs go to the local nursery and we often walk with another mum and child on the school run.
We normally chat and get on but her child can be a bit bossy/rude and she never says anything. Stuff that I would always give a warning about - teasing with toys/snacks, shouting, saying 'you're not my friend' etc. Even to the point the nursery workers heard and said 'that's not nice'.
AIBU to say something next time? I tend not to say anything to other peoples' children but mine are well behaved (and if not I tell them off!) and I don't want them to think it's ok, especially when the mum doesn't say anything.