When is it Abuse?

When is it considered abuse? I mean does it matter if it's your one child abusing your other one? You see, tonight Steff had it out for Sadie...Sadie ended up with a bruise on each cheek, they are in different spots on each side so they weren't from the same episode.
This is what I know...One minute Steff is in the kitchen doing whatever and trying to wake up from her nap...which she actually was half way in the kitchen and half way in the hallway when she fell asleep and I, being such a wonderful mother, left her there for the entirety of her nap. I figured she was taking one so leave her alone...She doesn't transfer well at all, so, if I had tried to move her, she would have woke up and stayed awake...
Anyway, Sadie crawled into the kitchen and apparently she got too close to Steff because when I came in, after hearing Sadie start to cry, I found Steff down the hallway, hiding from getting in trouble and Sadie smack dab on the right side of her face...
Later, the story is kind of the same, they were in the kitchen again and Sadie started crying and Steff ran off to hide. This time I'm really not sure what happened since Sadie was just sitting there crying and Steff took off to hide...I know one thing though, Sadie ended up with a bruise on the other side of her face. Kieran said Steff hit Sadie but that's all the information I got out of him...
I just had a revelation or two...Steff always hides when she hurts Sadie...and, 90% of the assaults happen in the kitchen...
I know I'm in for much more of this and that's something I've come to terms with but I'd hate to get in trouble for something my kids do to each other...