Okay, I went back and watched it AGAIN (yes, Denise, I did watch it before commenting--obviously I didn't see it correctly. But to be fair, everything did happen pretty fast.
But, Goat, may be onto something. Maybe the big kid did think the little boy was cutting. Like I said, I don't think they (D.S) think/comprehend the same way we would expect "normal" folk to.
And I would hope if the dad did realize the person who kicked his kid had down syndrome that he wouldn't have hit him (as much as he would want to).