Every meal needs a bread part - garlic bread or rough balls with pasta dishes, naan or chapati with curry, tortillas with chilli etc. All really easy and cheap to make and will help fill them up. Side dishes are brilliant for making meals feel huge.
Start having pudding every evening. Massive fruit crumbles with custard, sponge cake with ice cream, canned rice pudding, apple strudel, jam rolypoly etc. All can be made really cheaply (buy the frozen strudel - it's really cheap). I noticed a huge reduction in the amount of food we used when we started doing this.
Freeze homemade pizzas - about 20p each. Keep back a few spoonfuls of sauce when you have pasta and top with that.
Freeze loaf cakes in slices for lunches - the stodgier the better eg gingerbread, banana loaf, carrot cake, apple cake. Use all soft fruit and veg up like this. Also freeze muffins (sweet and savoury) and flapjacks.
Serve out huge portions at mealtimes but make sure you do it so you can then serve portions directly in to freezer containers. These then become microwave meals for after school.
Big blocks of cheese from aldi/lidl are much cheaper than any 2 for £5 offer I've seen. They're £3.50 ish for over 800g.
Make your own bread rolls - you can then make them huge, they are way more filling than sliced bread and they can also go (ready split) in the freezer.
Dcs can then take a roll, fill and it will be defrosted by lunch. Add cake, fruit, crisps, pizza slice etc and that's their lunch. It will defrost fine by about 11am.
Fillings can be cheese, humus, coleslaw etc.
They must also eat everything at the table. Makes you feel full sooner!