Also, you didn't ask, but I don't think I'd give up time at the Grand Canyon to spend time in Joshua Tree in August...Like Death Valley, you won't be able to hike much, August conditions can be too hot even in the morning if it is humid (yes it can be, monsoon season). There is a loop road with pullouts between Joshua Tree and 29 Palms.

https://www.nps.gov/jotr/planyourvis...ad/jotrmap.pdf

I hadn't think in the route on the north of JT in place of the south. Seems a lot better. Thanks!
I don't think about hiking in Joshua Tree, just drive around, make some photos and that my family see what is a real desert like in the western movies (we're Europeans).

Well you are definitely going to see a lot of that! In Death Valley from the road you can see in the distance some big sand dunes, Joshua Tree has interesting boulder formations, and the joshua trees and cactus gardens.

A good website about deserts is www.desertusa.com, you can read about the Mojave and other deserts there

http://www.desertusa.com/mojave-desert.html#

On your trip take a short detour from I-40 to Seligman which is on historic Route 66..If you hadn't already planned it, make a quick stop at Delgadillo's Sno-Cap.