If it's not a matter of affordability, I say feed the parents too. I've shown up to a few parties where the food is for kids only and I always feel it's a little cheap, especially if I'm expecting to be fed and I skipped lunch for the occasion.

As far as fun, I don't know what kind of climate you're in, but when my kid was that age (in Texas) we'd set up the slip and slide or order a bounce house and we'd be set. The kids would mostly eat and run around and bounce in the bounce house and stuff.