At a small soft play with my 3 YO today when I saw a boy turn round and push my DS by his face down the steps. Shortly afterwards the same boy pushed another younger child down the same steps (who tumbled into the child behind him and they both land crying in a heap).
So I approached the boy's mother and told her what had happened.
She smiled and replied "Oh I don't think so, it's not like him" and turned back to chat to her friend. As I was staring at her in surprise, she then said sweetly "I'm not worried because I know he wouldn't do anything like that".
I then removed my DS as I didn't want him to get hurt. As we're leaving I see the same boy grap a toddler by the hair and shove him down the same stairs. The toddler's DM started shouting at the boy, who ran off. The boy's mother was still chatting and didn't even look up at the shouting.
AIBU to be amazed that anyone would be so utterly oblivious to their DC's behaviour. Wouldn't you at least get up and check on/with your DC if you got an inkling they might be misbehaving? She clearly believed I was lying and that her DS was an angel. What would you have done?