We've done it twice, with two kids, age 7 and 6. The first time was in July, and we stayed on site and had FOTL, and with their ages and heights, couldn't do the big coasters. We were tired at the end of the day, but didn't rush through and had a great time.
We just did it again in November, with another family and their DD age 7. This time we were offsite, but bought the express pass for US. We also did the coasters, trading off the kids. Again, very manageable. We didn't run and rush, but again were tired.
Both times, it was a lot of fun at the end of the day to count up the rides we did- it was amazing how many rides we went through in one day. Our schedules both times demanded just one day for both parks, but in the future, I'd love to split it up in to 2 - mainly that you just went in to sensory overload by the end of the day. We were numb to 3-D, and truly couldn't stand to be jossled or jiggled anymore.
You probably won't be able to fit in all the shows and do all the rides. Also, I would recommend starting in IOA. The walking was very manageable for us, and I'm sure with teenagers will be even easier.
I wish we were going back with you- have a wonderful time
