What do they eat at school that they like? If anything like my kids you can’t go wrong with pasta, pizza, pies, curry, quiche, nuggets all really easy to do. If you have space in the freezer, batch cook a huge bolognese sauce, literally 5 cheap ingredients (pork mince is way cheaper than beef and less fat, chopped tomatoes, onions, garlic, herbs) and then freeze portions you can quickly defrost and add to cheap pasta.