Camping Sites

Dave in Queensland there are many approved rest areas where you can stay legally for 48 hours in any one calender month.
You can download the maps and other information from this website:
(Copy and paste URL into browser)
http://www.mainroads.qld.gov.au/MRWEB/Prod/Content.nsf/DOCINDEX/Map+Products?OpenDocument

In N.S.W. you can download this PDF map of rest areas. Not anywhere near to the standard normally of Qld ones, but they are improving rapidly. (Copy URL into browser)
http://www.rta.nsw.gov.au/roadsafety/downloads/fatigue_roadsiderestareas.pdf
and this one which is printable:
http://www.rta.nsw.gov.au/roadsafety/downloads/rest_area_mapfinalart.pdf

We always travel with good maps and find places off the beaten track next to rivers or dams etc. Major problem at the moment is the lack of water everywhere. If I have seen a good place to camp in a forestry patch on a farm, I have always asked the farmer and always been given permission. In N.S.W. it is also legal to camp in N.S.W. forests and of course then there is all the National Parks in Qld and N.S.W. which often have free or reasonably priced camping.Check N.S.W. as the 'reasonable' in many cases has come out of the pricing.

Dave, dunno which route you'd be taking heading up, but Easter last year, a mate and I camped on a stock route on the banks of the Balonne river just out of Dirranbandi on the backroad to St George. Was a nice spot with exception of the flies. But given you have 3 littlies with you, this spot may not be an option and seeing as water is an issue, this spot may not be as good as it was. I have the co-ords written down somewhere, so if you want them just let me know. cheers, Sam.

I'd be a little bit careful about rest areas as the Queeensland Government under pressure from caravan park owners has gone all out and closed most of the rest areas to overnight camping. Have a look at this site as it is an update on Camps Australia Wide and see how many rest area's have been closed. http://www.campsaustraliawide.com/Updates/updates.htm
I think be a good idea to get a copy of this book and the updated before you go as it may beneficial to you.
If you have a look at this site you will find some interesting caravan parks and some farm stay type accomodation, one I noticed was around the Carnarvons that seemed to get good reports.
http://www.geocities.com/rainforest/3648/parks
Another advantage is get a copy of the Queensland Caravan park book from the RACQ as it has a lot of caravan parks in it but from my little experience there are many good small parks that you will not find in any books and you just seem to stumble on them or you find out from other travellers.
I know I haven't answered your question but I hope I have pointed you in the right direction.