Where are you coming from? If you're going through Vegas on your way to the Grand Canyon, save some time to stop off at the Las Vegas Area Council office. It's one of the better BSA offices in the U.S.. And if your boys are into geocaching, I have a geocache hidden at the council office called On My Honor
. I also have a bunch of caches just off I-15 heading north out of Las Vegas all the way to Zion NP. Finding them would break up your trip a little.
I agree with Mike that Toroweep would be a great place to take your boys. The roads should be dry as we have had very little rain and no snow this year. The North Rim would be a little cooler and have trees for camping, but Toroweep would be prettier and more isolated. Here's some pictures from when Mike and I were at Toroweep:
Have a great time with your Boy Scouts. And thanks for your service in turning young boys into strong men of character.