eggs work really well in my house- and making scrambled eggs is as quicj as porridge. I add some grated cheese to it and each of my boys eat a couple. sometimes they have them in a soft wrap with some avo and tomato. sometimes just as it is
We also do soft boiled eggs and soldiers some days - but i need more time for that

I also make bircher muesli (its quite quick) some nights for the next morning - which they love.

Also 'banana toast' - mashed banana and cinnamon on wholegrain/rye/seeded toast. this is a huge hit.

Pancakes are also a good option in our place - i make the batter the night before and just cook a few for them (this is a treat though, as not as filling as other options)