whats a photo card because i need to know to get child fare on public transport?

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i want to know what photo card is because i might be going to winter wonderland and im going to be taking public transport and it says on the website that you need to be 10-15 which me and my friends are but theres a catch it says that i need a photo card and i need to know what this is so i can get child fares heres the link for you guys and girls to check out if you want more info to answer my question and i would appreciate it very much if you could help me out http://www.londontoolkit.com/briefing/underground.htm