https://tfl.gov.uk/fares-and-payments/travel-for-under-18s/travelling-with-children#on-this-page-0
Under 11s
Free travel on Tube, DLR, London Overground and TfL Rail services
If your child is under 11, they can travel free if:
-They have a 5-10 Zip Oyster photocard, or
-Travelling with an adult who is using pay as you go, or has a valid ticket (excluding Group Day Travelcards), a Freedom Pass or an Oyster photocard (up to four children per adult)
​十一岁以下公共交通免费，所以不用买oyster card