It’s around five hours by car from London to south Devon, with the M5 a good, generally fast route. Within the country, roads are being improved all the time and pretty much everywhere is easily accessible.
Devon is about three hours from London Paddington with Great Western trains. Virgin Trains runs two routes into Devon from the north-west, north-east and Scotland. If you’re coming from far away or are very short of time, you might consider flying to Exeter.
A car is only really viable way to explore Devon in depth with kids.