If you can, find out the name of the guest and address the invitation to both your friend/relative and the name of their guest. But if you don’t know and can’t find out the name -- and email asking should suffice -- “and Guest” is fine.
I remember learning that “and Family” was a no-no but I think times have changed, especially if space on the outer envelope is limited. Whether you’re having your envelopes calligraphed by hand or machine ask the vendor if it would be possible to fit parents and children on the outer envelope, and if you can then do so. If you can’t “and Family” won’t get you in trouble with the etiquette police.