Etiquette for joint parties?

[thaliablogs]

We are invited to a party where we know one child very well, and the other not at all - they go to a different school etc. What do I do re a present for child #2? They are only 5 so not likely to really notice individual presents, although I guess the parents will!

If you don't know the other child at all, I think it would be perfectly acceptable to get a present only for the child you do know. Parents won't mind (you wouldn't expect people coming to a joint party you were holding to give presents to your kids if they didn't know them I assume?)