I was at a local cafe this morning with a soft play area. I had DS1 (3.5y) and DS2 (12m) with me. There were four other children at the soft play, all around 3-4years old, and as their parents were sitting together, I think they knew each other.
In the hour that we were there, one girl (similar age to DS1) snatched everything my 3.5y/o picked up. When he tried to play with equipment, she would push in front of him so that he couldn't use it. She also blocked him from entering the little wendy-house and told the other children not to let him in. She was nice and inclusive to all of the other children.
DS was very polite in return, saying "please stop" and often finding something else to do if she wouldn't let him play with something. I said "we can all share" pointedly at a few points, and encouraged my son when he was being polite ("well done DS, it is good to be kind"). At no point did this girl's parent say a word to her about her behavior, and none of the other parents were really watching their children.
The time I chose to remove DS from the situation was when the girl went up to him and whispered something and my DS burst into tears. He came over to me and I gave him a cuddle and he said she told him that "your mummy will go home without you and leave you here all by yourself." I couldn't believe a 3-year-old would say something so malicious. At the same time, DS may not have got it entirely right, so I said "right, we're going to go home. You have been a really well-behaved boy, DS." We left and I told DS that sometimes people aren't very nice, but it's important that we are kind.
Should I have stepped in and said something to either the girl or the mum? I feel awful for DS, but I didn't want to overstep. What is the correct course of action here?