I have no idea which section to post this in, and no idea why tfl are being so coy about it, but I had an email from them saying trains will be free for kids age 5-10 with zip cards from 2 Jan. Yay!!
Email says:
"From Saturday 2 January 2016, travel on most National Rail services where pay as you go is accepted will be free for children with a 5-10 Zip Oyster photocard. This is in addition to the existing free travel they receive on buses, Tubes, trams, DLR and London Overground.
Free travel will not apply on Southeastern high speed services between St. Pancras International and Stratford International, however they can use their 5-10 Zip Oyster photocard to pay as you go at the child rate on these services.
Please note children must have their 5-10 Zip Oyster photocards with them at all times when travelling on National Rail services. "