Just as all children are different, we all have differing opinions, but I couldn't let this one pass. So I'll address the youth question first and boat questions second. And I know there is a lot of I/me here, but I knew many similar friends in my youth, and as a NJROTC school teacher have known many since (in "modern times."). Kids that, with encouragement, can do WAY more than you might think. And if you teach them safety and demand they follow the rules, they'll survive.
By the time I was 9 yrs. old I was building tree forts and "things that floated that resembled boats" with hand tools, and "experimental" bikes & wagons & etc. off of our "junker" scrap heap, and hunting with "adult" firearms. And overnight camping with friends by ourselves, no adults present. By the time I was 12 I was operating and maintaining the lawnmower, a (smaller size) gas chainsaw and a little 120 cc dirt motorcycle. And right at about 12 or 13 I started on an experimental aircraft kit (a "PDQ2" - Dynel cloth, epoxy, powered grinders, saws, precision measurements, 2-cycle motor, etc. all involved) that I had to cajole my dad into purchasing. Of course my focus on the project wasn't at the adult stage, and school and teenage jobs interfered, but by age 17 the plane was done.
Bottom line, let Dad judge the capabilities of the kids, and after the first several weeks when the "newness" and focus wear off, if they aren't focused on the project today, they'll get their focus back tomorrow - and the cycle of focus will repeat. Either work forward on your own, or wait until they are ready to get back to the project (while gently nudging them as to their committment). In my opinion you're doing the kids a great service for their future lives to get them involved in something like this now!
I do agree that 3 persons in either boat will be about as many as you want for sailing. So teach the kids to sail (and row) AND SWIM and buy them a life jacket and turn them loose. I didnt mention above that in 6th grade my little brother and I came up with half the money, my grandmother the other half, and we got a brand new 6ph Johnson outboard for our jonboat. I think the total price was $600 at the time. We lived on a lake in Michigan. Getting that motor was much better than when I first got my drivers license. The motorboat was "freedom" - no more riding bikes to town, we could take the boat - little league practice was a stone's throw way from a place to park the boat, and the Tastee Freeze nearby.
As to building outdoors, that will be doable but a challenge. I think you'll need a 20 ft. long carport sort of canopy. Also, if you've got no floor to lay things on, you'll want several long tables end-to-end for initial stages of butt-joining the planks & etc., and then when the hull moves onto saw horses, the tables (which can be kept under the canopy when not actively using them) can be moved off to the side (even if not under the canopy in good weather) for all the "table work" you'll be doing during assembly, with the pieces then being added to the hull on the saw horses. You certainly will want to cover everything with a tarp every night, even under the canopy, to keep the dew off. You'll need some relatively dry, non-humid days for varnishing or painting, just choose wisely.
You can do it!
