Nestled on the border on the Magic Kingdom's "Tomorrowland" and "Fantasyland" areas is one of the better quick-serve restaurants, Cosmic Ray's Starlight Cafe.
For years (it's pretty much become a routine), we've regularly eaten a quick lunch at Cosmic Ray's at least 2-3 times per Disney World trip. It's in a good location (as we spend much time in Tomorrowland), and it has a good selection of food. It has its burgers, salads, chicken,.....really a good variety.
Sure, it's all theme park food, but you definitely get good portions of all your food. It's not the best food you'll ever eat, but if you know how bad typical theme park food can be, I will say it's a higher quality than what you're probably used to. I usually get the cheeseburger (or bacon burger) and it's of good size and comes with a ton of fries (which are also good). But there are also vegetarian offerings and as I said, a variety of options.
Of course, it is somewhat pricy if you're going in there and are NOT on the Disney Dining Plan. At least with the DDP you get your meal, drink, and a dessert altogether. And trust me, you'll more than likely not finish everything they give you because you'll be full by the time you're halfway through dessert.
Just go in not having the highest of expectations and you may be pleasantly surprised. At the very least, you can have your kids still be entertained by "Sonny Eclipse" and his "space angels", an animatronic character who sings and tells jokes. Trust me, the little ones eat it up.
It really is a good fast service place to get yourself a quick meal before racing around again through the park or wherever your next destination will be.