Quick background story...
My Mum had a housemate with a 6 year old daughter who were living with her up until 2 weeks ago.
We pop in regularly and the twins (4yo) usually have a sleepover on Friday nights. I haven't liked this girl after I caught her yelling at the girls that she was going to "Boot them up the bum" for standing on the front gate. Words were exchanged and I thought there were no more issues.
Today the girls said "We better not go in there or C will smack us on the bottom again", I asked them about it and they said they'd gone into her bedroom and she smacked them. The way they said it and because of the other incident I most certainly believe them.
I am SO angry and not sure how to confront her over it. I'm not sure when it happened but I'm not going to just ignore this, I've spoken to the girls and said if anything like this ever happens they are to tell us straight away because it's not on.
My first thought was that I'd like to see how she likes being smacked but obviously that's immature and two wrongs don't make a right.
Any advice on how to ask her about it?