What is the cheapest way to travel around London?

I will be staying in zone 4 and wondering if I can get a daily travel ticket? Also travelling with a 12 year old and a 16 year old. Is there a family ticket I can get? Also I've heard of oyster cards. How do they work if I have a daily ticket?|||I think its under 16 to go free not 18. but they have to get an Oyster card when they are 11 or 12 its free of charge I think they just need identification eg date of birth and I think a photocard which Im not so sure about.|||If your staying in zone 4, then your on the outskirts of London, the best way for you too travel would be buses or trains. Depending on where you want to go, oyster cards are ideal as you can top up or put 拢7 pounds which last the day there for adults, but with kids i'd suggest buying a family travel card from a news agents or ticket office in the underground which could save you money. Unless they've got the under 18's travel card which means there free on all busses and only pay 50p per train journey. Hope that helps