Just make them decent simple food. Nothing wrong with a pasta dish, hide veggies in the sauce if they're fussy.
Packed lunches, make sure you have zero prepacked things, so no crisps, no cereal bars, no little bags/boxes of things etc.
Ham/cheese/marmite sandwich (not rolls, too expensive)
Fruit - they like melon? That's cheap fruit.
Some veg they will eat, say a few chunks of cucumber or. Cherry tomatoes or whatever.
Buy or make a larger "loaf" cake, and slice it up for packed lunch.
Cheese - slice/cube it up, don't use things like babybel or small snack size packs etc.
Yoghurt - buy a larger tub, and decant it into small tupperware.
If you know they don't like minced beef on its own, but they'll eat Bolognese for example....give them Bolognese. Or serve it as mince, mash and veggies. Some kids don't like "mixed" food like this.
If they eat things like tomato soup, that's really easy to pass off most homemade soup as "tomato soup" ,. especially if you just make veggie soup with odds of veggies and colour it with tomato puree.