"Best" is very subjective. Do you need to be flexible with timings, or can you travel on a specific train. Does it matter how long the journey is? Can you book in advance or do you need to book on or near the day of travel?
The cheapest through tickets from London to Truro available for 1 adult, 1 child with a Family and Friends Railcard cost £12.85. These are Advance fares which have limited availability and are only valid on the booked train.
The cheapest though "walk-up" fares are for 1 adult, 1 child with a Family and Friends Railcard cost £47.60 (single) or £82.90 (return). These are Super Off Peak tickets which are restricted as to when you can leave London M-F but don't restrict you to a particular train or route you may take.