Some good ideas for drinks at a kids luau include Hawaiian Punch or any fruity punch that has some sugar involved. Water, juice, and maybe pineapple milkshakes are also possibilities.
How about Shirley Temples or a smoothie bar? Kids can mix together ingredients like orange/pineapple/mango juice, frozen bananas/strawberries/papayas, etc. Top with cocktail umbrellas and pineapple pieces.
Some of these might include Hawaiian fruit punch, coconut slushies, "virgin" pina coladas, pineapple juice, fruit punch slushies or pineapple slushies.