What about a breakfast casserole? You could have everything prepared the night before and then it just needs a few minutes in the oven the morning of, or bake it the night before and reheat in microwave the next morning. I do this all the time...I usually make a large casserole and then put it into individual containers for my SO who can then grab one to go in the morning for at work (he has to be in early).
You could also do cereal...you can still get those little boxes of cereal (variety of flavors) at the store. Yogurt and granola, egg scramble w/sausage or bacon and veggies, waffle sticks (we have a waffle stick maker and little dipping cups but you could use a regular waffle maker and cut them), bagels and flavored cream cheeses (strawberry, blueberry, etc) or cream cheese and jelly (my son's favorite). I would be careful of food allergies, make sure you know if anyone has a peanut allergy before you serve peanut anything (and don't trust the younger ones to tell you...my son tells everyone he is big enough to eat peanuts). SUPER CHEAP - Cinnamon Toast: regular bread toasted, with butter...sprinkle with sugar and cinnamon...I use to eat this as a kid allll the time. SUPER CHEAP - Rice cereal: Regular white rice heated, pour milk over it, add brown sugar and cinnamon. And of course you could always do straight Oatmeal...which is always a super healthy way to go and can give them much needed energy and sustain them until lunch or snack time.
Good luck...we are forever having the same problem in our house...I don't envy anyone who has to do it for a lot of children.