July would be peak season for Simpson crossings, so even if something was to happen I don't think help would be too far away. (You will be outside school holidays so you may have to wait a few hours, instead of minutes - well maybe I exaggerate slightly).
We were in a similar situation in 2000, when friends baled out part way into our trip (Ni$$an broke). When we rolled into Dalhousie it had been 42 that day (late sept), we decided that if the forecast cooler weather didn't arrive we woudn't do the crossing solo. Next morning was MUCH cooler, so we spent a great 3 days meandering across the desert.
We had hired a HF set for peace of mind, kids were between 8 and 11.
Go for it.
(Comments for the Simpson section - no experience with Gunbarrel bit)
Martyn

