What about mock pina coladas? You could get some funky glasses and those little umbrellas. Here is a good (alcohol free!) recipe:
1/2 cup chopped pineapples (fresh or canned) 1/4 cup nonfat milk 1/4 cup water (or more) 2 tablespoons sugar 1 drop rum extract 1/4 teaspoon coconut extract ice cubes
Place everything except for water and ice cubes in blender and blend until combined. Gradually add in water and ice cubes until it reaches desired consistancy.
Other things to serve could be fruit salad, kid kabobs (you could supply skewers and pre-cooked things like ham/chicken cubes, cheese, veggies, dipping sauces, etc. and have the kids each make their own) , BBQ, anything with coconut (coconut shrimp?), hawaiian pizza, macadamia nuts.
Sounds like a really fun party theme! Can I come?