With our kids at the same ages we enjoyed - The 50's Prime Time Diner and the Sci Fi Diner at MGM (the theming is pretty heavy at both of these). Hoop Dee Doo was fun but it is pricier! One of the best tips I read was to go to the restaurants late in the afternoon and just share appetizers and drinks or desserts - this way you can experience the restaurant without all of the $$$. Character meals are always a hit with my kids. OUr kids alos enjoyed the Rainforest Cafe (Downtown Disney or Animal Kingdom).
In March we were able to get walk in reservations for 3:45 for the Sci Fi Restaurant. However, that is an odd time so I think it wasn't too busy. I would make PS just to be sure. You can always change them and if you have to cancel someone else will gladly take your spot!