In Holland, children under 4 travel free. This ticket is valid for 1 day and can be used in 1st or 2nd class, dependent on the accompanying adult ticket. Up to 3 children can travel with 1 adult with a valid ticket but each child must have a Railrunner. On international journeys children under 4 travel free and those aged 4-11 pay half the adult fare.
Details are available from UK stations.