Today I saw a 2 year old (big, heavy) child on top of my (lighter) 4 year old in the soft play centre. The 2 year old was doing a twisting / pinching type thing on DS's bare arm, like a chinese burn. My DS did not retaliate but tried to get up to walk away, but the 2 year old grabbed him round the neck with both hands and tried to pull him backwards to the floor again. He would not let go, even though DS was trying to get towards the door to get out.
At this stage, I said 'no', but the kid kept going. So I got hold of the 2 year old's arm to physically remove it from DS's neck and then I put myself between the two of them. I then bent down and said "no, don't hurt him" to the 2 year old, and I still had hold of his arm, because he was trying to hit me.
Whereupon the father of the 2 year old, (who hadn't seen the original incident but had been attracted by me saying "no"), shouted at me " don't squeeze his arm or I'll do it to you".
I said I was sorry, but that I had to intervene because his son was hurting mine badly and he had been going to do it again. But he would not let me speak. So I went to sit down. As I went, I heard him say "oh did she hurt your arm?" to his boy, and the kid burst into tears. 
Then the man came over and started shouting at me over and over that his son was only 2 years old, and that I had squeezed his arm and bruised it. (I am absolutely certain that I did not hurt or bruise him. What a joke.)
I said to him "I have listened to what you have had to say to me, but could you listen to my response now because you are not letting me speak at all?" but he just kept saying "he's only two, he's only two".
In the end, I said to him loudly that "your son was hurting mine badly, but instead of telling him off and dealing with it, you have come over to have a go at me, how dare you?". But he just kept on shouting at me..
I am so upset. I am re-playing it through my mind non-stop and want to have a large glass of wine. I know it's just a silly spat, but this bloke was about 6'ft 4".
The problem is that this is the second time this has happened to me, in very similar circumstances. The first time was two years ago though. Is it me? Should I keep calmer when I see a little thug laying into my kid? Or do I have to accept that there are parents who will let their children do anything and will shoot the messenger who complains?
AIBU, or was he?