I think you'll be fine, because after all, it's Disney World, and the 13 year old will have loads of fun. As far as hanging with the young ones, with that many adults, you should easily be able to have someone peel off the larger group and spend some time with the older boy. He'll want to ride Space Mountain, Big Thunder Mountain Railroad and Splash Mountain, which most of your young ones won't. I'd also recommend some of the things that take up more time for him. You could go over to Tom Sawyer's Island with him and let him roam around through the cave. You could spend some time at the arcade in FrontierLand and shoot the guns. There's a fun little area in AdventureLand that has little boats you can remote control. I think if you build in some of those kinds of activities, you could have a pretty happy camper.