Chalfont and Latimer is in Zone 8, and Central London would be assumed to be meaning zone 1. You should buy a Zone 1 to 8 travelcard covering all National Rail*, London Underground, Docklands Light Railway, London Buses, London Tramlink within Zone 1-8. It gives you 1/3'rd off travelling on TfL River services as well.
Travelling Off-Peak (from 09:30 Monday to Friday, all day Saturday, Sunday & Public Holidays) this will cost £11.00 per adult, your three year old won't need to pay.
If a child is with someone who has a valid ticket, is using Oyster to pay as they go, has a Freedom Pass or has a Veterans Oyster photocard the child gets free travel on buses, Tube, trams, DLR, London Overground and National Rail.
* Not valid on Heathrow Connect between Hayes & Harlington and Heathrow and not valid at all on Heathrow Express.