I honestly don't think the walking is the thing. Kids play rough.....it's normal. I just would never chance it. I thought you had said 5 lbs...but yeah, 9 isn't a whole lot different.
If the parents have not figured out what happened, I don't think you will either. Just be thankful you have gotten a warning. I have seen dogs with fractured limbs...and have seen some dropped who died.
Children really don't mean to harm as a rule...but, they just do not understand. They want to pick them up...hug them, smother them...many of them end up getting bitten and the dog ends up being labeled a "biter". I don't mean to sound harsh about children....some really DO know how to handle little dogs but in my experience it is a rarity. My granddaughter LOVES dogs...but I would never leave her with mine. Actually one of mine did bite her one day.
The other problem is that children run as I mentioned before and are not looking for a small dog underfoot.
I am glad your pup is ok!