I agree that it seems crazy to have to extensively plan a beach vacation! I messed up our character breakfast (booked it the wrong day) and now the day I want is not available. And that is not until June! Crazy. Per my reading here, you have a decent chance of getting a character dining reservation one or so days before as a walk up. We will try this if I can't find an online cancellation.
With regards to Aunty's, I think their system is terrible and as you said, the pre registration is not well advertised. Another option for childcare that we are using on our Pearl Harbor day is the babysitting through Aulani. I didn't want them in the kids' club all day, so we are getting a sitter. It might be an option for you. For us to pay for one of the premium activities for our two kids was going to be about the same cost as a sitter for 4 hours. If you have more than two kids, it would actually be cheaper to get the sitter. The sitters can't take them in the pools or ocean but can take them to other things around the resort. I haven't reserved the sitter, but I'd better get on that since everything else seems crazy booked. Here is the link to babysitting:
https://resorts.disney.go.com/aulani-hawaii-resort/rooms-offers/in-room-options/childcare/
Another thing to remember, there are options other than the cabanas for reserved seating that no one can book in advance. The casabellas are reserved day-of only, first come basis. So if there is a designated "pool day" for you, maybe one adult can get up early to get one of those? Maybe a day early in the trip when you are still on mainland time and getting up early anyway due to time change? Just an idea from someone who has never been there yet (but I've been reading a lot of great advice here! ). Also the family resort activities are also same day sign up, so no one can pre book them out from under you. It does mean an early trek to sign up, but not 0630 early, from what I've read (someone correct me that's wrong).
Have a wonderful trip!!