I don't have children yet, but I do have a 5 year old sister, which is like 20 years younger than me, I know, I know, my mother...
Milo at first played a little too rough for my liking, but so did she, they also did not always get along, he would steal things from her, stand on her, and attempted to "chastise" her quite often. My suspicions were that he thought that she was lowest in the pack order. My sister was three at the time, but I would give her treats and have her try and make him do something as simple as making Milo sit, supervising and advising her on technique. I even corrected her tendency to cry when he wronged her, teaching her not to be the victim, but trying instead to tell him NO and then telling me so I could further correct him. We also began to go on walks together, I would allow her to hold on to the leash, showing her how to hold it firmly, and I would take it for the hard parts like crossing the street, at least until we got to the park. This worked wonders for them, and in time they became very good with each other. Even at three, I think kids can be taught to handle a rough pup, it is just about supervision and establishing the pack order.