I echo what everyone said about menu planning. Look for cheap but nutritious meals, some of which you can vary in many different ways.
First of all, check what you already have in the house and try to base some meals around that.
Things on toast - baked beans, perhaps with a little grated cheese; poached or scrambled egg; cheese on toast
baked potatoes - again, a variety of fillings/toppings. Beans, cheese, coleslaw, grilled tomatoes (or just heat up a can), grilled or fried mushrooms, tuna mayonnaise
omelette
quiches and pies - if you make your own pastry these can be really cheap. Flavour with a few cooked veggies (even just frozen peas and sweetcorn) or cheese and tomato or cooked lentils. A few herbs will boost this.
corned beef - one tin can feed all of you, either something like corned beef hash or in a corned beef and potato pie
pasta - buy a big bag that will do several meals. Try simple sauces - for example, soften some onion and perhaps some carrot or pepper, add in a tin of tomatoes and some herbs and simmer until nice and thick. Or do pasta and tuna or just pasta and veggies.
pizza - home-made pizza is gorgeous, and very cheap. Either make a bread base or a scone dough for the pizza base, then add whatever toppings you've got in the house.
scones - home-made scones make a great lunch or tea
mince - usual suspects like shepherd's pie, spag bol, lasagna, meatballs. Pop in a few lentils or oats if you want to 'stretch' the meat.
Lentils - red or green lentils cook really quickly, are dead cheap and delicious. Look on the internet for recipes or try your library or ask on here!
Soups - gently cook a diced onion for 10-15 mins, add in whatever combination of veggies you can find cheap, add some stock and cook for 20-30 mins. Liquidise and add more stock or water or milk or wine or whatever if you want it thinner. Serve with cream crackers, scones or home-made bread.
Your children are very young so won't really know if you stop bought treats for a while. Concentrate on free activities to get you through this month.
For the adults, if you have a video recorder keep looking in the TV listings and tape any films you fancy. Then treat yourself to a 'cinema at home' night. You can even do home-made popcorn if you like!