Use all, if possible, food you already have. Veg soups, veg curry, chopped up sausage/fish/chicken/whatever's in the freezer/fridge with pasta/rice/couscous/potatoes. Bulk meals out/replace meat with lentils, buy brown rice- it takes twice as long to cook but you only need half as much and it knocks white rice out of the park in terms of nutrition. Do you have anything like canned fish, canned soups etc?
If you normally throw the last bit of bread out start using it for bread crumbs. Use these as topping for baked food, or thickener for homemade soup. Or slice/chop bread and make a pudding. Any stale cakes can be used as bread crumbs, add to chopped fruit, yoghurt, desserts.
Could you do packed lunches instead of paying for a while? Plan meals for a week, or longer, using only what is in the house right now. Give DCs water to drink instead of buying your usual alternatives. Don't buy any treats, most homemade cakes/muffins etc freeze well so make a batch to last for a week.
Do DCs help themselves, mine drive me nuts with this new discovery of theirs, if so put a stop to that. Is DH also concerned, enough to back you up when you explain that they are to ask about all snacks etc before taking it?
Make a "fridge soup/salad/pasta/baked potato" basically whatever you can chop up and and turn into a sauce/filling/topping. Slighty tired looking veg makes a good curry. Ditto fruit, either chop and use raw in a salad or bake.
Whatever you buy on the way to/from work, stop buying it and take something from home.
HTH