Just a footnote here on the physics that might be helpful in this situation. The stuff that makes it "hot" is IR length radiation - the right length to jiggle the molecules in your body efficiently. If you create an IR barrier you may run the risk of trapping it in as well. This is what happen in your car on a sunny day. Furthermore if you block the sun, the material you use will eventually heat up itself, and will emit IR, not to mention heat the air. Long story short, don't use a tent for this. Use a tarp, assume it is going to heat up and pitch it as high as you can so that some of heat captured by the tarp will get convected away before it gets to you. The breeze, if any, will help you as well as the tarp to be cool.
So, shading surface as far away from you as possible, and no enclosed spaces. Using a tent is more or a less a guarantee that the inside of the tent is going to eventually get nearly as hot as the surface of the tent. If you are planning naps as part of your family trip, maybe take a specialized awning tarp for this purpose. Pr4obably on of those heavy duty space blankets (or more than one hooked together), the kind with grommets would be ideal for this. Extra weight, but maybe making your family go UL with you is a bit like making your dog go vegetarian. :-)
If you already have natural (high up) shade available then that is about the best you can do from the temperature standpoint. Then just do the minimum extra required for getting the child comfortable.
If you have to use the tent, say, to keep out the dingos, then shade the tent in the same way - high up shade awning shading the tent from a distance.