You can go from robe right thru to Pt McDonnell on the beaches / dune tracks, from pt mac you can also keep heading east to the vic border.
We have done this trip several times and must warn you, the beaches east of carp rocks can be trecherous. I would recommend atleast 2 other vehicles as backup, and be sure to know what the tides are doing.
Its a great drive but caution needs to be very carefully considered.
Call into info center in Robe. They will help you with maps, tide times, current conditions on beaches/sands, etc., and give you lots of good safety information. If done properly as mentioned above by others it is a lot of good fun. If the going looks tough just head back to the road.. Have done Robe to Beachport a couple of times, just give yourself plenty of time.
When in Robe, go for a drive along Long Beach in town and have some fun driving on the sand. Then you can air down and experience the soft sand without getting into trouble. Long beach runs for about 10 k's or more and is great for family picnics, etc.
The town of Robe is great for family outings also. I reckon I might be up there again early January..
Great camp ground at Cape Jaffa just north of Robe, run by our esteemed member "Grumpy" .