Formal, please. But, we want you to feel comfortable, so if your shoes are too tight and your buttons are about to pop, let it all hang loose.
If your "RSVP" included your date, then yes, if not we prefer just you.
We love your kids, but we want you to have a good night also. But if by any means a babysitter is not available the kids are welcomed. Don't forget to include them on the "RSVP".