Lakeland do toastie bags that you just shove a sandwich into and then chuck it in the toaster. They're supposed to last 500 uses, but we had a set that lasted a LOT longer than that. They're pretty fail safe as long as they go in the toaster with the open end UP at the top (you can work out why I stress that, I'm sure).
Tupperware boxes of homemade soup such as a chunky broth in the fridge - easy for them to fill a mug with it and reheat in the microwave. If you divvy it up into recycled takeaway boxes, it's even pre-portioned.
Homemade nachos - a small packet of lightly salted tortilla chips, a grating of cheese and a spoon of salsa (mild or spicy), melted for just a few seconds in the microwave (the chips burn easily so they need watching). Add a bit of sour cream and guacamole after.
Quesedilla - dry frying pan, heated. Small tortilla, lightly cover with a bit of grated cheese, some salsa, maybe some jalapenos and some ham, top with another tortilla. Put into pan and cook for a minute or two till brown in spots. Flip over and cook the other side (will take less time). Slide onto a plate and slice into triangles.
Quick stir fry - have a bowl of pre-chopped, prepared stir fry veg in the fridge ready to go. Shove on the frying pan, a spray of oil and throw in a few handfuls of veg with a handful of cashew nuts and a splosh of soy sauce, some sweet chilli sauce and a blob of honey. If they're super hungry and dinner will be a couple of hours, they could always add some straight to wok noodles too.