It depends on the pub - some don't serve food at all and some have a whole restaurant section. Also, not all pubs allow children inside - usually pubs that serve food are family-friendly (e.g. Wetherspoons), but some won't let anyone under 18 in.
But general British Pub food includes:
Burger and Chips
Pie and Chips
Roast Dinner (usually on Sundays)
Jacket (Baked) Potato (with various fillings) and Salad
Bangers and Mash
Ploughman's Lunch
Hot Pot
Pasties
Sandwiches
Salads
The more restaurant-based pubs offer a wider variety of food (including children's menus), such as:
Pasta dishes
Curries
Panini's
Stir-fry
However, if you're looking to eat cheaply, I'm not sure pubs are really the way to go. While they're generally cheaper than restaurants, the food is still fairly pricey compared to the US and is getting more expensive as time goes on (
http://www.guardian.co.uk/money/2007/oct/16/lifeandhealth.foodanddrink)