Desolation is spectacular. And there are other ares that are even more spectacular, and over a larger area, and see fewer visitors. Will you have a great trip if you go to Desolation? I bet you will. Will you have a great trip if you go to any of the aras that have been mentioned in this thread so far? I bet you will. There are so many places in the Sierra that are beautiful, spectacular - any adjective you like - that it's pretty hard to go wrong.
Desolation is a fairly small wilderness are with good access from all sides. What this means is that there is a lot of it that can be seen by day-hiking, so you see quite a few day-hikers in various parts of it. This is a big part of why it has a reputation for being crowded. Overnight campers are limited by trailhead quotas, but dayhikers are not, and since it is not a large area, there is not a lot of it that is out of the reach of a reasonable dayhike. And the most beautiful parts - Desolation Valley, Pyramid Peak, Gilmore Lake, Mt. Tallac - pretty much all fall within dayhike range, and thus see quite a few dayhikers during the prime season - which is when you will be there.
So, I have a new suggestion that has just come to mind, and I think it would be great. Blackcap Basin on the west side of the range in the John Muir Wilderness.You'd start from Wishon Reservoir in the Sierra National forest, go by way of chimney Lake, Crown Pass, and Halfmoon Lake, up to Portal Lake or Lighting Corral Meadow. Now you're in Blackcap Basin, right around treeline, with a bunch of lakes to explore to, Blackcap Mountain is a walk-up with big views, and the ridge on the est side of the basin, the White Divide, offers a number of saddles and peaks you can scramble up to for truly SPECTACULAR views over into the rest of the range. You might even have time to go over the ridge or around the corner into the next valley north, Bench Valley, for more great stuff.
The more I think of this the more I think this would be ideal for you and your scouts. Grander than Desolation, less traveled, and they'll really get a feel for how big the whole area really is if you get them up to the white divide so they can see over to the east into an ocean of peaks stretching off into the distance in all directions.
If you look online, you can find many photos of the area.
It does take a little more driving than some other spots, but man, it's worth it.