To ask your absolute cheapest ways to feed 1 adult 1 5 year old

Agree lentil and carrot soup with some bread.
Butter beans, onion, chopped tomatoes, courgette with some rice
Pasta, onion, dried basil or oregano, few mushrooms. Grated cheese if you have some.
For cheap pizza use a wrap or Nan bread, chopped tomatoes as a base then some mushrooms and grated cheese
Sweet potato baked with cheese and baked beans
Omelette

carrots and cheap and filling,

other veg varies, you can just go to the supermarket or green grocer and see what is cheap today.

Bananas is another go-to for me, when skint..

Porridge with full fat milk is a cheap and filling breakfast. Big cartons of natural yoghurt, with any cheap fruit is a good dessert. raisons go well with either, and can sometimes be brought cheaply

sausages - well, some are better value nutritionally than others.

eggs, a bit expensive, but a good source of protein

Cheese - massive differences in price - look for multi buy offers as it lasts

Rice, particularly brown rice, is much better for you then pasta - can be flavoured with onion, spice, etc.

lentils can be very cheap

Don't forget green leafy veg, lettuce, cabbage, etc very important for a healthy diet

Have you a freezer? Going off root veg (courgette, obergine, carrot, onion, peppers, tin of tomatoes - anything left over - OR often found in lidl/morrisons £discount wonky veg boxes) chop finely, oven roast and use as a “healthy base” for all other meals (I add garlic and dried herbs but it’s not needed). Mine will have it over pasta. Potatoes and as a “dip” for other dinners (mini pizza etc). Can be made in a big batch and frozen/defrosted in a microwave and just stirred into meals as needed.

Meal planning is your friend - if you have the storage space. I use chicken thighs a lot - shred the meat. First night (for adults I’ll add tomatoes harissa or any other dried spices and a veg cube from above) with beans (and plain rice) and eat in wraps. Second night I’ll add it to rice cooked with a stock cube. Frozen peas and broccoli.

Join the olio free food app.
Buy seeds from aldi/lidl are start growing veg
Buy wonky fruit/veg
Cut down meat and replace with lentils/beans
Lots of similar threads if you look

Browse too good to go. The grocery and local grocery type stores with 4.5 or higher ratings can be very good

Pasta . Tin of tuna . Tomato ketchup .
can add frozen veg too

Bean chilli is pretty cheap. You can add some veg as well - carrots, onions, etc.

My DS basically lived off it at uni.

You can get big bags of cheap pasta as well and do tomato sauce with lentils/carrots/other cheap veg

Also stir fry rice - big bag of rice. Cook the rice, then fry with whatever veg are cheap. Eggs if you can stretch to them for protein.

lentil curries with added frozen veg is very filling and delicious. Have with rice

Omelettes with pitta bread

both meals have carbs and protein but are cheaper than meat

Lentil soup is brilliant -tasty, cheap and healthy

Have a look at full time meals with Tom Kerridge on instagram