H.P.
Home made personal size pizza using english muffins or the small tortillas
"Ants on a log" - celery sticks with peanut butter and raisins as the ants. Watch out for nut allergies though.
Spinach Brownies - yes there is actually a recipe out there for them. I found it in a magazine. Unfortunately I've moved since then and the recipe did not get into my recipe box. They didn't taste bad either. The only thing you have to watch out for is chopping the spinach fine enough so they don't find it. If you get a stringy piece of spinach it's a dead giveaway.
You could let them make healthy "sundaes" by making a parfait of yogurt and fruit.
Best wishes on the party!