I just read in the growing baby literature that when children hit other children at this age (nearly 15 month old), it isn't aggression but their curiosity. They want to see reactions. I don't believe this. When Millie smacks Jo, or Jo pushes Millie out of the way, I think they are irritated with each other. They spend every waking moment, literally every moment of their day, together. They inevitably get tired of each other. Or at least tired of competing for toys, space, and attention. I'm not sure what's going to happen when the new sibling comes along!