Is the train a better way to travel rather than drive to the south of france from folkestone kent?

That depends on how much time you want to set aside for it.
By train you get there overnight, with you relaxing on a bed through the night.
Driving you will need most of 2 days, and you will be driving most of the those days.

If you have the time to set aside 3 days, and take your leisure, traveling by car can allow you to see something of France.
But if you just want to go to the south of France, the train is a good bet.

By train you can get there the same day, thanks to Eurostar and TGV. Leave Ashford on the 09:26, arrive Marne la Valee (aka Disneyland Paris) at 12:27, leave there at 14:09, arrive Avignon 17:09. From there the train continues to Marseille, half an hour later. Depending where in the south of France you want to go, you should be able to make connections the same day.

Then hire a car if you want to drive around the area.

Train is probably cheaper for one, but car might be cheaper for a family.

Yes train is fantastic , i did both train down then car back , see more and different experiances.