We are spending 6 days in London and staying just outside the city limits (zone 3). Our children are 11 and 13. I am so confused on which travel option to purchase! 7 day travel cards bought at a train station get you 2 for 1 entry at some places. Can you use a RAIL travel card on all the same transports as a travel card bought at an Underground station - bus, rail, underground? From what I understand, you need to apply for and pay a fee for a child photo Oystercard 4 weeks in advance in order to get the child rate on an Oyster card or travel card bought at a tube station. But it seems like you can just provide a passport photo and show a passport to get a kid rate travel card at a rail station. Does anyone know if this is true??? So very confusing....
If you want to use the 2 for 1 discounts, then you need a ticket with the national rail logo (the "crows foot"). So you wouldn't want an Oystercard because those are issued by TFL (Transport for London). Both the National Rail travel card, and the TFL travel card cover the same transportation: underground, light rail and buses. I'm afraid I'm not familiar with the rules for the child rate travel cards since it's been quite a while since we traveled with children. You also might want to check the websites of the places you plan to visit and see if they offer family rates: a flat fee for 2 adults and 2 children. It's probably not a better rate than the 2 for 1 deals, but it's a way to save on places that aren't part of the 2 for 1 scheme.
Think of it as kind of like buying a club membership with reciprocal privileges. You can buy a week's worth of rail travel from National Rail in London with TfL thrown in or a week's worth of TFL with National Rail thrown in. Each sell according to its own rules and practices. The only advantage of the 11-15 Oyster card is that you can get free bus travel and more heavily discounted pay-as-you-go on other services - but you don't want that for other reasons, even if it might work out cheaper for travel.