Carshalton Beeches to Victoria would be on National Rail which doesn't accept Oyster Cards. The best option is a Travel Card. You would need all zones which costs £7.50 providing you travel off peak (after 9:30 or at weekends). You can then use any form of transport all day. An adult with a Travelcard can take up to 4 children (aged 5 - 15) for a flat fare of £1 each off peak.