We just returned from London and just wanted to pass on our unsuccessful attempt to get an Oyster Card for our 13 year old son. We followed instructions and emailed our request for an application well in advance of our trip (we started the process for it in April for our Oct trip). We received the application in May and submitted it and were debited for approx. $10.00 in June. We thought everything was settled and went to pick it up at Victoria Station on the date we requested but they said due to a clerical omission on our part, the application was not processed. Their application states monies will not be returned if there is an error. Lessons learned is to wait for confirmation from them the application was processed and the Oyster Card would be at the place you requested. Although it took an hour and a half to stand in line, explain, and find out why the Oyster Card wasn't processed, it wasn't a big deal in the larger scheme of things - just a little disappointing.
Is perhaps also worth pointing out that purchasing Oyster Cards is not a particulary good deal for children in London for many visitors.
Children under 11 travel free anyway on public transport in London and there is a day Travelcard you can purchase for all children under 16 that costs 1 GBP per day (about $US1.70) that covers all public transport. An Oyster Card for a child is 1 GBP per journey to a daily ceiling so its is several times more expensive. Also if the parents purchase a Travelcard too (instead of an Oyster Card) from rail station you get 2 for 1 admission offers at many of the main sights like the Tower of London and the hop on, hop off buses as well as restauarnts etc. These savings for a visitor can easly pay for the transport costs.