Sounds like you are doing this trip for all the right reasons, and you are all going to have a blast.
A few random notes...
- You already know the weather can be variable, but Death Valley is great for access to a range of conditions. Too hot? Head for elevation. Too cold? Valley floor and lower canyons. Wind? Tuck up into a canyon mouth to find protection.
- Your iten sounds great. My suggestion would be to budget time, at least two days and maybe three, for the swing through the northwest part of the part. It is remote, beautiful, and empty. There is also lots to do that the kid will enjoy, like the racetrack, abandoned mines, and of course the dunes.
- IMO, the backcountry camping options blow the doors off of developed campgrounds in DV. With the exception of Wildrose, I find the campgrounds very depressing, although I have never stayed at Mahogany Flat.
- You mentioned the 'no backcountry fires' rule...yes, I know it is a rule, and I don't advocate lawlessness, but...there are plenty of established fire rings, and it really elevates the mood to have a fire going during the evening out there. Bring your own wood, of course.
Seems like you have a good read on keeping the kid entertained. In my experience, he will have a blast when outside the car, but long desert drives are very boring for kids. They just can't appreciate the subtlety of the landscape. Audio books are a great idea.