You've seen them before. The foil like structures in some cars...otherwise known as sunshades. I finally broke down last week and bought one. I couldn't take it anymore...the heat that is. My inside car thermometer read 156 degrees...and my steering wheel was so hot...I thought it was going to take a layer of skin when I grabbed.
They really do work. The theory is...is that they reflect the heat....radiation and light back into the atmosphere instead of your car absorbing it. These shades can reduce your car's interior temperature by at least 20 degrees...and prevent fading and cracking on your dashboard..
When they first came out..most of them were only made of cardboard...but the recent technology makes sense...with a foil side to these gizmo's. It was so hot the other day the foil started to bubble. Either it was really hot (more than likely) or I bought a "lemon". I think it was the first thought.
In any case it's a good investment you can find them in most automotive stores.
Ending on a serious note...NEVER lease your kids and pets in cars under any circumstances during this heat spell. Cracking the windows makes little difference.