Details.
Some train operators offer flat fares for children travelling with adults, with return tickets available from £1 and some fares including a London Travelcard. Railsmartr offers accompanied child discounts for c2c, Chiltern Railways, Govia Thameslink Railway, including Southern, Great Northern and Thameslink, and Southeastern. These discounts are normally available only on Off-Peak and Super Off-Peak tickets. Everyone must travel together, otherwise an undiscounted fare or a Penalty Fare may apply.
