Hi - I've got some useful travel tips on the website I edit - here: #mce_temp_url#
It's the entire Transport for London website - condensed down into a few paragraphs.
In your case - I'd get a weekly Travelcard zone 1 and 2 (not Oyster) - approx £25.80 for an adult and £12.90 per child - valid for 7 days, unlimited travel, anytime in zones 1,2 (everywhere central).
When buying - purchase them all at the same time as discounts are usually available on child passes when bought in conjunction with an adult.
Patrick