I have only been to one 1st Birthday party and the Birthday Girl had no idea what was going on. It was a morning tea and they had tea/coffee/juice and food for adults and the kids that were there.

It was more really for the adults but they did have a kids decorated cake and handed out lolly bags to the kids.

I think it depends on how many kids are going to be there as to how you make it, though as the Birthday Boy/Girl probably won't know what's going on and will get tired soon into it then I would try to keep it a bit low key.