If it is something your family has always dreamt of, you should go for it! Book the cheapest possible room since you won't be using it much, if at all. If you want to see Aulani, play in the pools, float in the lazy river, meet characters, snorkel in Rainbow Reef, and play in the kids club, it would make a brilliant family excursion! Since it's a cruise ship excursion, you wouldn't really need the room itself anyway, so the later check in time won't matter too much. Once you have your room key, as soon as you check in, you can get bracelets and towels to enjoy the guest-only activities, characters, and pools. Bell services can hold onto any of your belongings if you have a bag with stuff you won't need until your room is ready.
Many locals will spend one night and check in early to get two full days at the resort, so you're not alone in this idea.
Reality: it's probably going to cost the same or less as many other full-day excursions from the cruise ship for a family, so I don't think the price is too much. You will have to pay for transfers and meals separately, plus any extra activity costs (snorkelling, paddle board rentals, etc.) Aulani pools close early in the winter, so after sunset and dinner you should have plenty of time to get back to the ship. I would plan to leave Aulani no later than 8:30 PM, so you can be back at the ship by 9:30 PM, with an hour to spare in case of traffic delays. (They will likely tell you to be back by 10 or 10:30 PM if the ship is scheduled to leave at 11.
This is such a brilliant idea I may recommend it to someone else I know who is in a similar situation.