We stayed in a studio at Aulani as two adults, and thought it was crowded. I can't imagine more people than us in that room, even kids. It was the fourth resort we had stayed at during our time in Hawaii, and it was the smallest room by far. With your party, I would do the standard room for two queen beds. Otherwise, it will be one queen bed and a sleeper sofa that you'll need to pull out and make up each day. If you leave it in the sleeper position, there will be little to no walking room and balcony door access will be tough. There's also limited storage space in the room, IMO.
PP mentions the benefit of the studio is the kitchenette, which is true. But the amenities of the kitchenette can also be found at Ulu Cafe for the public - microwave and toaster. I believe all rooms have a mini fridge. It's of course not as convenient as having it in-room, but the only time we used the kitchenette amenities was to heat up leftover pizza and make pop tarts, both of which we could have done at Ulu.