Should i have said something?

maybe there is no correct answer. . we will all think something of it! as long as we see it, but the "normal" thing is to do nothing, unfortunately. there are infinite strangers in the world, and our unifying feature might be that we don't want no unwanted trouble. this makes life easier, but also allows some people to be cruel and go down life without having to change.
now, change is not always for the better.. if you had said something to the parent, it might have made him treat the kid even worse. like ofc, we would hope people are not that stupid, mean-spirited, etc, but you never know. it ain't easy to see the error of your ways, and it's much harder to act in order to be better (especially if it is recommended by a frigkin stranger) most people would just assume they've done nothing wrong. they are the victim, and who the fuck are you to say something when you have no idea what is going on in their lives?? you think you understand the situation, just by seeing a moment??? basically, there is no correct answer. nothing one must do in order to help in such a situation.
I could suggest a couple of ideal-like options tho:
1) try to draw the attention to yourself. maybe approach the parent for something unrelated, or if you're nearby pick up what has fallen and laugh. say it happens all the time. maybe knock something off yourself, if you're being extreme. just do something.
2) approach the parent about it. but try to be understanding. hold empathy not only for the kid but the parent too, considering that abusers are usually also in pain if they are hurting someone important to them. of course it is their fault, but being understanding and warm will make it easier to approach them, and should allow them to listen to your words (hopefully words of wisdom, about peace and love or something, or how we're all blessed-)
3) just run away. if you dont like the situation, you can run away, and that would draw some heads too so why not.

Daaaaamn. Calm the F* down.

I KNOW IT WAS AN ADULT TALKING TO A KID BUT ADULTS WERE ONCE KIDS. IN THIS CASE, YOU COULD NOT WIN UNLESS YOU WERE WILLING TO COP A POTENTIAL MOUTHFUL FROM THE ADULT. PERHAPS IT WAS JUST AN ACCIDENT, YOU SAID, YOU KNOCK THINGS OVER TOO. IS THE STORE CLUTTERED? IS THERE NOT A LOT OF ROOM TO MOVE? OR WAS IT JUST CROWDED.

HOW DID THE CHILD REACT, SEEM FRIGHTENED?

BTW. SHOPPING WITH KIDS IS A NIGHTMARE.

It's not up for you to judge the parenting of others unless it's child abuse. The vague threat of violence made does not constitute abuse to the point where I think you should intervene. Its not that you shouldn't care, it's that you shouldn't care enough to stand up his mother in public. What would you have said anyways out of curiosity?