You can do it very easily .... Potatoes and pasta are your friends. For oily fish, I NEVER buy salmon, only mackerel (which is delicious) and meatwise, mince and cheap cuts like shoulder of pork and shin of beef are much cheaper.
With regards to chicken, portions are cheaper than whole chickens and if you don't gorge yourselves on meat, which isn't a healthy thing to do anyway, it can be a healthier option as the dark meat of poultry has a much higher iron content.
Don't buy cod and haddock. It is nutritionally inferior to pollack and coley, and ethically unsound because of the appalling overfishing. It is also, and I freely admit this to be the main reason for my boycott, twice the price of pollack (the so called 'omega three white fish') and three times the price of coley, which is just fine in pies.
Eggs make a good quality, easily-got-into-small-mouths form of protein, and are also cheap.
All this also depends on the size and appetite of your children - my three year old eats like a bird, my 6 year old eats as much as I do.
Ev'nin' all!