hi Jason
September out from Cairns is the dry season and I would expect ugly, flat, treeless scenery once 300 mile out. Suggest you investigate. Take plenty of spare water and emergency food. Not supermarket eaty salty bites
Remember that way is isolated country, largest cattle ranch in the world, towns are small and uninteresting. Need good employment to pay for the diesel
Later I will send URLs of areas. I have been the highway and boring, but then, it is better than busy cities and lots of adventure if children are that way inclined. Take the Nintendos
Some URLs
http://www.exploroz.com/Places/29368/QLD/Karumba.aspx
http://www.cloncurry.qld.gov.au/community/community_photogallery.shtml
http://en.wikipedia.org/wiki/Mount_Isa,_Queensland
http://www.totaltravel.com.au/travel/qld/tropicalqld/cooktown-cape-york