Most of the kids in Surf's Up were in the 4-7 range. There were a few older ones, including a girl who was about 11 and looked, honestly, miserable when we picked up. But, if the group skewed older- it would probably be fine- hard to say for sure. And that was just our one experience, so I am not sure if that is the typical mix.
You can also reserve your kids a spot for dinner. You have to line up at 7AM to do so (or I did in June), but it is only like $8 a kid and they had an array of kid friendly options to choose from. If you are signed up for dinner, you are guaranteed entry into Aunty's 30 minutes before dinnertime and then they can stay as long as you/they would like. You can drop off anytime in general, but we did see the "at capacity" sign a few times. If you are signed up for something (meal or a class- free or paid), you will still be let in. At night there was a comedy free class that sounded fun- especially for older kids!