I have never once seen my (now 5 yr old) be aggressive around any of my grandchildren and they climb all over him.
I can only think that because yours is still a puppy, he is playing like they are litter mates. Possibly like a dominence thing and because they are large, even as pups, they do not realize they are big. Has he/she been fixed? I honesty have never owned a more gentle and loving dog in my life and I have always had dogs of many different breeds.