hi mommy trixie.
welcome to the forum.
for a hawaiian or luau party, its usually connected with the beach or sea animals. you can ask your guests to come in beach attire. you can prepare kid's leis or flower head clips to greet them, instead of party hats.
as giveaways you can give them sunglasses, flip flops (there are a lot of affordable ones), beach balls, etc.
for food/drinks it would be good to just prepare finger foods since the kids would be going in and out of the pool. and add colored juices to make it fun.
for activities, maybe you can have the limbo rock, hula hoops contest, even hula hoop statue dance, and find the sea shell. and how about a pinata shaped like a fish.
have fun and happy planning!