Don't know if this is what you're looking for, but I just thought I'd share what we do for dd. We make sure that she always eats extremely healthy at home. Like, she never gets any refined sugar products at home. Ever. We only keep stuff in our house that I feel good about her eating.
So considering that, it's okay with me if she eats cake at a birthday party. Even if we were to have 3 parties in the space of a month, I would be okay with her eating junky stuff on those occasions. I just always make sure she knows that cake is only for special occasions. I think it would be very hard for a little one to see everyone around her eating pizza and cake and be told that she couldn't have any. If the occasional treat is really not acceptable, then I would be very wary about accepting invitations to birthday parties.
I think it is a good idea that if you are hosting the party (or if you have some input about what is served) then you can obvioiusly make the snacks healthy.