V.
It's probably a good idea to follow up just so you have a good idea how many kids will be there. I don't think it's that big a deal that 1 family said they didn't get an invitation or can't be there. That will happen, not everyone will come, and sometimes invitations get overlooked or don't make it to the parents. You could always send a second notice to try and remind anyone who forgot and inform those who missed the invitation. I really don't see the big deal if she has plenty of friends coming.