Denise,
OK, I am assuming that they want to eat somewhere in or near the Magic Kingdom? Assuming so, here is my suggestion. While you two are doing the castle thing, I would have them leave the Magic Kingdom and take the boat over to the Wilderness Lodge Resort. There they can have a great (and fun) lunch at
Whispering Canyon Cafe! The entertainment is perfect for a 7 year old, the western setting is very boy-friendly, and the food is really quite good!
I strongly recommend the Canyon Skillet!
Make an Advanced Dining Reservation, just in case. As for time to eat at Cinderella's Royal Table, it should closely match the time it takes the boys to get to the Wilderness Lodge, eat, and get back. All told, 90 minutes, including some waiting time. Could be faster, but probably won't.
Have a magical (and delicious) vacation!