I made one of the chairs in my shop with power tools only takes about 30 minutes.
But, I think 1x material is not going to work.
Every DIY site I see calls for 2x12, I think 2x10 will be fine. Also most plans call fo 48 inch parts. I made this one 40 inches and it worked fine, but 48 would be better.
Since you generally buy 8 foot material anyway, might as well go with 48.
So, I recommend one 2x10 pine 8 feet long for each Scout.
No, prep work, the troop dads will cut the 2x10 in half on a miter chop saw.
I wish we could use a portable table saw for the long rip cuts, but not sure about that part. The kids really should rip with handsaws. It may take too long in 2x material.
We may also want to bring some battery drills and Forster bits to speed up the mortise.
Best plan may be to set up stations and walk then through. Layout, ripping, crosscut, drilling, chisel, maybe spoke shave to ease corners and smooth long cuts. Since ripping and chisel will take longest time would be good to double up or even triple on those.
Would really like to let each cub leave with a camp chair.