Just because it's lunchtime doesn't mean they have to have lunch there. As long as there is availability for kids they can go in; they just won't get lunch. Aunty does provide snacks (free), so they won't starve.
Actually my DGD was there last night until 5:40; Auntys serves dinner at 5, DGD got crackers and milk for a snack, then she was picked up and had dinner in the villa.
Depending on when exactly your spa apt is I would give the kids an early lunch or late breakfast. (and try to bring them down early to make sure there's room, if they are not signed up for anything)
*FYI: If you submit your Auntys pre-registration online it will ask what programs are favored, then email you with what your kids are signed up for (1-2 of them).