Should we buy a pass for the tube? We leave tomorrow from Colorado for London for eight days and have a firm commitment trip out of city on Sunday. We want to take one more day trip to Bath/Stonehenge probably.
Get an Oyster card at any tube station for the most cost effective travel. Preload it with whatever you think you will need. You can always top it up if you need more.
Since you are in London for eight days, I would purchase a seven-day Travelcard instead of using Oyster pay-as-you-go. It will save you money compared to the daily Oyster cap, and it also means that you don't get charged a higher fare for travelling before 9:30 am, which you will on Oyster. A Zone 1-2 weekly card will cover most areas you're likely to visit, but the Travelcard will be given to you on an Oyster, and you can put PAYG money on the same card to cover any journeys further out.
BUT if you want to take advantage of the 2 for 1 offers:
If any of your party are students/children I believe that you'll need a passport sized photo for the Travelcard. Getting one before you go will save you time and trouble when you get there.
7 day travel card = photo required (for everyone with a travel card).
1 day travel card = photo not required.
If the travelcard is put on an Oyster, you do not need a photo. Have you been on the tube website www.tfl.gov.uk to get any information? Depending on what ages your children are, they may be able to ride for free or a reduced rate.