Take cell phones or 2-way radios so that you can occassionally split up (provided that they're both mature enough to make that feasible). Neither is absolutely necessary, but they'll help put your mind at ease. Phones will allow you to be in separate parks. With 2-way radios, you won't be able to go as far away.
Make sure that they both have watches. There's no excuse for being late if they both have watches, since there's really no chance they'll both break at once.
Have fun! They should probably both be allowed to make some of the plans, or at least a list of the things they don't want to miss, if they're interested.
My first trip to WDW was with my best friend and her family when we were 14. We had a blast. We did split up from her parents a few times, and sometimes we took her 11 year old sister with us. We never did anything that we wouldn't have done if we weren't with her parents (except ride Space Mountain and Haunted Mansion about a hundred times each). There's so much fun stuff at WDW that there's really no need to do anything wrong.