First- congrats on going to Hawaii- very exciting and fun!
If you have the 2BR option, go for it. That way you and your husband can have some privacy and the kids can go to bed at their normal time.
Call the hotel to find out what equipment they have- very often they will have cribs, rollaways, and sometimes bed-rails. You shouldn't have to bring that with you. You may need to reserve these in advance- do that right away.
As far as getting them to sleep- just stick with your home routine as best you can. For us, it's bath, jammies, brush teeth, story, bed. Simple. Kids take comfort in routine, and the closer you can stick to theirs, the better. The first night they will likely be so pooped from traveling that they'll crash without a fuss- the second night they might notice that the scenery is different but by then they may not care. Just don't call any attention to it- make the differences something to have fun with- it's an adventure!
Let them pick a few comfort items from home- a few small board books, a favorite stuffed animal, blanket and pillow. Kids are incredibly flexible and resilient when you give them the right tools and it seems that you're doing the right thing, thinking about this right now.
As far as carseats in a taxi... never had any experience, but if it was me, I'd grab a minivan-taxi and try to use the carseat. Check with a cab company or a travel website for Hawaii!
Good luck, have fun, and don't forget the sunscreen!
K.