I remember years ago as a CM at soft play, a little girl made a bee line for my 4 charges who were about 1, 2, 4&4. She was about 4 as well. She pushed over both twins who were small children, slapped the 2 year old and although I was sitting next to the baby and I had asked her to please go play elsewhere turned and bit the baby.
I at that point took her by the hand and lead her to the from of the coffee area and in a very loud voice asked who's child she was. A woman at the back with a baby in a buggy, although the mother was on the phone looked up.
" could you please supervise your child, she has just bitten my baby and slapped my toddler " I had asked rather icily
"Oh I can't leave my baby" she replied
" Bring it with you" I had said even more icily
" oh she's asleep, if your down there you can watch her" was the reply as she went back to her phone.
At which point I went and leant both hands on the table - yes I was being intimidating-
" I am not your childminder, you do not pay me. Now go and supervise your bloody child or I will be complaining to the owners. "
She huffed and puffed called the older one over and left.
The owner came over then and asked what had happened. I explained, the owner then told me that she had had to speak to her the week before about her child hitting other children.
She had told her that her child had SEN, which explained why the behaviour from the child but not from the mother. If you know your child is going to react to other children like that surely you supervise even more.