I have done that section of the AT!
This may not too much for WEBELOS who have never backpacked, if you limit their pack weight by having a good gear shakedown, and limit their weight to about 25% of their body weight. This means others may have to help carry gear.
Or, you could let them join you for just one night on the trail! Have them backpack up with you to the first campsite. In the morning, they pack up and go back down to trailhead. This means you'd need two more adults, but they could ferry your cars to Crater Lake, and save you that logistical pain on the first day.