There's a national park outside Tucson that's named after the Saguaro and is full of them. As for hot yes both Phoenix and Tucson will be hotter than hades in the summer. I think Phoenix is a bit hotter tho.
In Tucson go to the Desert Museum which is really a desert zoo with lots of critters. Your son will enjoy it. I don't know if Old Tucson (it is I just checked) is still in operation but they used to stage gunfights and such there. Of course Tombstone does that for certain. I lived in Tucson for a few years when I was in elementary school.