We're just east of the Rockies in Canada - and can say it's really a matter of you and your family.
We'll be going to Aulani for our DDs 2nd birthday, then to WDW in Sept for our middles 5th birthday.
We've flown to England twice with a lap baby - once with a lap baby and a toddler. We've also flown to FL (same length of flight for us as HI) with one baby, 1 toddler and 1 baby, pregnant with a toddler and a preschooler, and with a baby, a preschooler and a 5 year old. Our first trip to Hawaii will have 3 kids with one on our lap. A block out seat is nice, but honestly we've only ever managed to get one once - and that was when we only had one little baby. We use a baby carrier (wrap or similar) for during the flight. When babies ready to sleep, I just put her on my chest and she'll settle right down. THough it does help that we put them to sleep that way at home too. We also bring a couple movies, books, (washable) markers. And we will walk up and down the isles when the carts aren't going around. It keeps the little ones happy and that lets them leave everyone else alone.
We did purchase a seat once for my one DD when she was not quite 2 - but the airline refused to let us use it for take-off and landing! That was very frustrating! We had a carseat too.
Once at Aulani we'll likely just stick to the resort (and marriot lobby), yes it might be nice to do 'more' but it'll be just as nice to relax at the pool, go for walks on the resort grounds, participate in the resort activities. Of course we also know we'll be back in 8 or so years. This is just a 'time to recharge' vacation and we don't want to do a ton of stuff.