I agree with Heather - there's been times where i have had to take all the kids to a birthday party - but i check with the host first, and make sure they're aware that they won't be participating in the party.

eta - if you're doing party bags, I often have a few spares (just plain brown paper bags) with just a couple of lollies in them. That way if there are a couple of extras - there won't be any tears (not that it's your problem though)