+1 for Murchison river...
It has become our overnight stop on a few trips now, travelling both north and south.
Possibly another option for you is to do as we have done (2 familys with young girls, not quite as young as yours) is to leave perth in the afternoon and drive into the night, we usually pull into Murchison around 10.30/11pm and camp up (quietly)
both times we have done this the girls fall asleep after dinner (at 440 roadhouse, just north of Gero) and dont wake up when we transfer them to the campers at Murchison.