I am so mad about what happened today

Today I took the kids swimming at a friend's community pool.
TC is very friendly and will approach kids.
Three girls and one boy were playing a game where they pushed one another in the pool and the last one standing was the winner.
I watched closely as TC played along. He went up to a small girl to push her and I told him not to, she was too little and he could hurt her.
Next thing I see was the boy going up to TC and grabbing his hands and pushing him hard into the water.
As TC came up, he was upset, I could tell from his expression.
I called him over to ask was he okay and he said, No his hand hurt where the boy pushed him.
I told him to not play with those kids anymore.

Later on, TC was standing in front of a boy (wanting to play) and the boy said, "You can't play with me, you are too fat!" I called TC over and told him LOUDLY because I have a big mouth..lol....that he didn't want to play with that kid anyways because that kid was being mean. I told the kid "You don't have to call him fat, he knows he is big but you don't have to treat him badly."
The Mom was right by the kid and ignored everything til' I spoke to her son.
She then called him over and told him that was rude.
No apology or anything.
I was pissed.

As we were leaving, approximately an hour later, the kid hollered out, "Sorry!" And I told Tc to tell him, "Okay."

My problem is why are kids so mean and why don't parents watch them? It is NOT OKAY to treat someone like they are a piece of crap because they are overweight, it just is NOT!!