Greetings,
We did what you want to do for years with a Ram 3500 quadcab. 3 kids in the back seat and it was small! They were fine.
I put a cap on the back of the truck with sliding/opening windows equipped with pet grates. Our 60 lb dog rode back there. I would open the sliding window to the back from the truck and had a wireless thermometer back there to keep track of the temperature. Generally, it did not get too much above ambient while underway.
I do live in the North East, so normally, it's much cooler than Texas though.
-Drew