We had to deal with this a few months back with Alex at daycare.
For about a month he'd be telling about a particular boy at daycare who would push him over, hit him (leave bruises) etc.
I let it go for a bit thinking that kids can usually sort things out & maybe he was making more of it than there was.
After one particular incident I went & saw the director of the center, thinking she would see me as an over protective mother, but she was great. Alex has been at the center since he was 15 months old so they know him very well.
He's a very soft, cruisy little guy.
They made sure Alex & the paricular boy didn't play together too much & kept me up to date every time he was there.
Thankfully the situation sorted itself out.
It's a scary thing to have to deal with.