For boys that age who eat nonstop, I would recommend a buffet. How about a late lunch? That would run about $18-$20 each and they could get VERY full. After that, a lunch buffet at Trail's End at Fort Wilderness is probably the next lowest cost.
Otherwise, they are ordering from the menu and probably the cheapest TS in a park would be the Plaza at Magic Kingdom.
Look above this page and click on Dining. You can click through to all the menus and they will give you specific cost.