travel card for trains,, pricing. help please. ?

What is the cheapest option to travel on the underground in london with 2 adults & 2 children,, does anyone know how much it would cost?or if there are group save options?.please & thankyou. x?

There are much more interesting ways to travel London than the stuffy underground.
This website provides everything you ever needed to know and a lot more besides!

http://www.tfl.gov.uk/

Get an Oyster for you and your wife.

Under-5s

Children under five can travel free at any time on buses, the Tube, trams, DLR, and London Overground as long as they are accompanied by someone with a valid ticket or who is using Oyster to pay as they go.
5-10 year olds

Children aged 5-10 years can now travel free at any time on buses, the Tube, trams, DLR and London Overground at any time.

http://www.tfl.gov.uk/tickets/faresandtickets/1063.aspx