Pizza Picazzo in Sedona is a fine choice for all age groups ( see: http://www.picazzos.com/), and the food better than any middle range priced fare you'll find in that gorgeous but overpriced berg.
As for South Rim restaurants, El Tovar's restaurant is really special, and I believe they're open for lunch, but if you go for dinner make sure you have reservations (and definitely go for dinner if you can because it's a wonderful experience). Bright Angel Lodge has a casual dining room (burgers, clubs, soups, etc.) that is fast and reasonable, no frills. If you can spend the extra bucks, definitely eat at El Tovar. The room is splendid, the food surprisingly good for National Park fare, the ambiance romantic and historical, and the service is first rate. El Tovar also has a handsome bar and lounge area, great for people watching and relaxing after a day on the trail.
Enjoy your trip.
I second the recommendation for Picazzo's...try the chocolate chip cookie pie (a donation from each pie is made to cancer research group and the pie is awesommmmmme!) A second Sedona recommendation is the Cowboy Club....great game pot pie, great steaks and chicken fried chicken steak, great cold beer and locals like to drink at the bar...we are talking real cowboys!