I definitely agree with letting the older ones go off by themselves. That way, they can do the bigger stuff if they want, they can stay in the parks instead of being forced to go back to the pool, and it's a safe place for them to experience some independence. And they can always meet back up with you. By the time I was 17, I didn't want to be with my parents 24/7 on vacation (heck, at 20, sometimes I still don't!), and when I was 12, I would have liked to have an older sibling take me around so I don't have to be with my parents (my brother is 13, I'm 20, and we go off on our own all the time). Just some perspective on the age.
Just an FYI, around 3 PM, the pool gets really crowded. Also an FYI, afternoon thunderstorms start at around 3 PM, so you might not get a lot of swim time in. Unless you are planning on doing rope drop (I know a lot of people like this method; I am not getting out of bed before 8 am on vacation) I recommend doing the pool in the morning and heading to the parks in the afternoon.