Since you've got a teenaged boy, I'm going to suggest a buffet - been there done that with teens involved

An added bonus would be that you can get food for your youngest without being charged for it. And for the middle kids, you could choose a character meal which may excite them more than just lunch would...
Crystal Palace at MK is a good buffet, and if you book the last 'breakfast' ADR time then they will be doing switchover to lunch while you are there, and you can eat a true 'brunch' with breakfast stuff and lunch stuff in the same meal. Pooh, Piglet, Tigger, Eeyore - these guys may be great for your 3-yr-old.
Cape May Cafe at the Beach Club has a decent character breakfast buffet too, they don't serve lunch. It has never greatly impressed us the times we've gone, but there are many here on the boards who swear by it so it may be just us, and turn out to be a good one for your group.
The Ohana breakfast at the Polynesian is also one of our favorites, and while the menu is limited to the basics - eggs, sausage, bacon, waffles, fruit, potatoes - the food is always good and the characters are a lot of fun (Lilo, Stitch, Pluto and the Main Mouse). They also do not serve lunch.
If budget is a main thing, you may wish to consider the Plaza at MK. They do not serve breakfast, but they do have lovely options for lunch/dinner with price tags per person some $10-15 less than other restaurants. Mostly sandwiches, burgers, salads - and lots and lots of ice cream.
HTH!
