You can also book Spa appointments in advance. If you're interested in the supplied air snorkeling (SAS) at Rainbow Reef that can be booked at least a couple of days ahead. We tried to book day-of for the 5 p.m. Wednesday session and they only had one spot left (there were three of use). They could've fit the three of us in the Friday morning session but we were leaving for the airport that day. So may be a good idea to check at least when you get there.
Dining is probably the most important, but its still not a critical "must book at 90 days" kind of thing. Spa reservations can also be booked ahead, but we just did it once we got there and knew what our days were going to be like.
Aunty's is only for kids ages 3 to 12, so your older one would not be allowed. Also, from many of the reports I have read, it seems like many of the activities are geared towards the younger kids. They also have other activities listed in the Daily Iwa for tweens and teens specifically, but those are not run out of Aunty's.