For those with "older" kids, at what age would you (or have you) felt comfortable letting your kids take on the parks on their own?
My son will be 15 this trip, my daughter 9. They've each been to WDW many times and know it all like the back of their hand. They know how to act and I see no reason why I should hesitate to let the two of them explore a park while we do something else either at the same park (or maybe the hotel if we were talking Epcot while we're staying at Boardwalk or somewhere close).
Not to mention, they both know that if one did something wrong, the other would jump at the chance to get them in trouble for it!
My son will be 15 this trip, my daughter 9. They've each been to WDW many times and know it all like the back of their hand. They know how to act and I see no reason why I should hesitate to let the two of them explore a park while we do something else either at the same park (or maybe the hotel if we were talking Epcot while we're staying at Boardwalk or somewhere close).
Not to mention, they both know that if one did something wrong, the other would jump at the chance to get them in trouble for it!
I will say, that when I was fourteen and my brother was fifteen, my parents let us go to MGM by ourselves on New Years Eve, and it was one of the best times I ever had. My brother was really nice to me the whole night. He even bought me a Tigger watch. I would think that your fifteen year old would do a really good job of looking out for his sister, if it was just the two of them at Disney.

Have a great trip and enjoy the time alone without the kids, they'll have a great time exploring on their own. I find with my own kids that the more freedom I give them the more responsible they are 

) while DH and I rode Splash (with a fast pass) or grabbed a coffee, etc.