its 4,000 km from Perth to Darwin, if you go flat out it's a three day drive. If you can average 600 km a day
with the family in a camper van 
you will do well, that only leaves 7 days in your two weeks for seeing the sights.
Do you have to do the run from Perth all the way to Darwin? if you don't have fixed commitments at either end you would be better off hiring your van and spending less time on the road by staying more 'local'. Maybe grab a van from Perth and do a loop around the south west or north towards Exmouth and back, or grab one from Broome or Kununurra and spend time in the Kimberley, or grab one in Darwin and take in Litchfield, Kakadu and Uluru, or even better fly to Exmouth, grab on of Turtlehunter's vans and spend two weeks seeing the North West Cape, Ningaloo, Pilbara, Karijini National Park, the list is endless.
A road trip from Perth to Darwin in two weeks is a waste, you will miss so much.
