Hi - the best option would be travelcards - one day. Adult zone 1 and 2 cards are £5.60 (off peak) for adults and £3.60 for children. Children under 10 years, travel for free, without the need for photo-cards - 11-18 and they need an Oyster Zip which is free, but an application form has to be filled out.
I'd recommend a combination of these cards, but buy them together as they often come with discounts.
More travel tips here: #mce_temp_url#