Well we have all probably been there....so what do you do?
A child around a year old was playing near my DD and was picking up and throwing things. Each time he did it his mum would say "no - you might hurt someone" and then hit his arm. Each time the kid looked puzzled then picked up the next thing and threw it.
After iteration 5 or so the kid burst into tears, which caused my 18 mo DD to do the same (she is going through an empathy phase apparently and does this every time another kid cries).
AIBU to ask what you do next on a scale of 1-10 where
1 is simply pick up DD and move somewhere else.
5 is saying loudly "don't worry DD, the baby just doesn't understand why it's mum is hurting it....and neither do I.
10 is hitting the mum while loudly explaining that hurting others is wrong.