Baked oatmeal is a great breakfast, for kids or adults. It has plenty of protein, and lots of fiber.
I've had success with a fritatta or quiche, or a breakfast casserole with a sturdy bread (torn-up bagels, French bread), cooked crumbled sausage, diced apples, then soaked in an egg/milk batter and baked.
I get the most bang for my food-prep buck, though, by making a batch of something that I can eat for lunch all week. This week I'm going to make this cabbage soup, but add canellini beans and maybe some kind of grain, like farro.
https://www.themediterraneandish.com/cabbage-soup/
I've also made grain bowls with brown rice or quinoa, roasted root vegetables, and chickpeas. It makes a giant batch that I can keep in the fridge all week. Each day, I heat a bowl up at work, add shredded kale and a citrus vinaigrette, and it makes a great lunch.