“Body slamming” *is* play behavior in all breeds.
However, your niece really should work on teaching the dog self restraint and what is and is not acceptable behavior.
While a *dog* would understand and appreciate that kind of play behavior, kids, not so much.
Dogs, being four-footed with a low center of gravity can easily accidentally knock down a two-footed high center of gravity person.
Your daughter was undoubtedly a novelty to the dog and he got overly excited by a ‘new thing’ in his world.
Odds are pretty good that he doesn’t act like that with her kids whom he sees all the time and is ‘used to’.
Any chance she would work the dog on-lead with your daughter or is she not particularly versed in dog training?
We live pretty far away and only see my niece about once a year. Having my daughter body slammed by a pit bull was unnerving. I never saw that behavior before from a dog, at least not that I can remember.