I have 3 sons. 5years, 3years and 9 months. I follow the "How to Talk to Your Kids..." method.
My two older sons like to rough house and wrestle and I let them. However if things get out of hand I stop them and they both have to sit where they are, breathe, calm down, and apologize (as it is always accidental.)
However, (and I know that they are looking for attention), when I place the baby down on the ground with toys and turn my back to grab the phone, stir cooking etc...one or both of the boys will attack the baby.
They squeeze him, and hurt him. (intentionally)
I have tried and tried and tried to talk about it with them, in the moment, later etc...how babies are fragile and one day they can rough house.
I just don't know how to make them understand that they could *really* hurt him.