Took my kids to mcd's to play cuz the park was too hot, and this 3 yr old kicked my 1.5 yr old in the face six times before I could get to her (she was up in the little level climber part behind plexiglass). Anyway, he kicked her hard enough that she cried for at least two minutes, and that's a lot for my dd, she is tough from wrestling and roughhousing with her bubba(whose also 3). So when I hit the plexiglass really hard right by his head to make him stop kicking her, he screamed at me and tried to push my dd again, so I put my finger I his face and very sternly told him kicking was not allowed and not to touch her again. And when I say "sternly" I mean i jumped his ass big time and made him cry. Anyway the mom didn't say nothing to the kid or to me, she just freakin sat there. And I'll admit I wish the plexiglass wasnt there cuz I seriously wanted to knock the freakin hell out of him. He was horrible the entire time we were there and was so damn mean, I finally told my older kids that if they saw him hit baby sister they had my permission to punch him as hard as they could. And I wish my oldest had seen it, she would've gone apeshit.
ETA: for all the moms bashing
me for my reaction, saying you would kick my ass and all that, here's the thing, for one youd be welcome to try and if I'm going to jail for assault then im most certainly gonna make it worth it. And I never actually touched the little demon boy, however if he had put his hands on her again then I would have very forcefully removed them and spoken to his mother.
And FYI my daughter had been in the little baby part where he had also been aggravating her and being obnoxious and since he was technically three he was supposed to be in the baby part too. So he wasn't separated from her due to age anyway.
And yes I teach my kids to defend themselves and their siblings against bullies, here's my Sibling motto "she's my baby sister, I can pick on her if I want, but if you touch her I kick your ass" that's just how it is.