Yes it is:
- Children under five can travel free at any time on buses, Tube, trams, DLR, and London Overground services as long as they are accompanied by an adult who has a valid ticket, is using Oyster to pay as they go, has a Freedom Pass or has a Veterans Concessionary Oyster photocard.
- Children aged 5 to 10 years can travel free at any time on buses, Tube, trams, DLR and London Overground services at any time.
Source:
http://www.tfl.gov.uk/tickets/faresandtickets/1063.aspx#Under_5s