Travelcards for London transport for teenagers?

I know it's short notice, but i want to go to London on Saturday ( the day after tomorrow!) and although I have an Oyster card, my 13 and 15 yr olds don't. which is the best way to get one day travel cards for them? On the day at a station? Or do I have to get photocards? Please can i have real advice from somebody who KNOWS, who lives in London, and please don't tell me to go the London transport website cos I've been and it's not very clear...that's why I'm here!