If I was boy b's mum I would have gone up to boy a and told him that my son was on there first and we can take turns. If he didn't get off I would have said to my child that we don't do that to people (what boy a had done) and walked off and played elsewhere.

I couldn't give a rats what boy a's mum would think because if she hd half a brain she would agree that what was done was unfair and if she was going to ignore or b***h about me because of it, i wouldn't want her in my life anyway