it sounds like you're talking about an older child hitting a baby who "doesn't know any better." that's not what i'm talking about - that's obviously not okay and not likely to happen. an older child hitting a baby is grasping at straws to try to explain.
2 kids of the same age who are, let's say 12-24 months old, ARE going to try to take toys away from each other. and when 12 month old suzie tries to take away 12 month old billy's toy - billy is going to smack, bite, pinch, scratch, or get a handful of hair at least ONCE! that's what young children DO. they don't know NOT to do that until they are taught not to do it, and you can't really teach them not to do it until it happens. unless of course they're required to watch a video that explains the importance of keeping hands to yourselves.
i don't care what kind of supernanny you are - you can not be hovering over every child EVERY second year after year and prevent the inevitable.