Sorry to bear bad news - if you're travelling on 'London Overground', thats from Clapham Junction round to Willesden Junction etc going clockwise, or to Canary Wharf etc going anti-clockwise its free. But on plain old overground services with South West trains or Southern yes you do need to pay for over 5s.
My kids have Oyster ZIP cards that they (you!) can load with cash that they use for those journeys Southern/South West train journeys and then touch in/out for free on buses and tube and London Underground.
Look at the last bit on the this link called 'oyster for kids'
http://www.southernrailway.com/tickets- ... ids-for-1/