Quick reply here since nobody else has chimed in.

Both the JS2 and NED are great designes that are well within the capabilities of a first time builder.  Both boats are too small for four people, especially while sailing.  Pictures are my GIS which is similar to the JS2 but 3' longer.  Sailing with 3 is the absolute max but 2 is better.

I love the idea of building something with the kids but honestly at their ages a boat is the wrong project.  Personally, I'd never let kids that age touch epoxy, power tools or anything sharp (saw, plane, etc) so that leaves little for them to do.  Also, at that age attention span is pretty short so after a day of two they would be totally over it.  I build model rockets with my niece and nephews that are about the same age, and 1-2 hours is all you get before they wonder off.  It will be another story in four years or so.

I think that CLC's advice regarding an outside build is sound.  It is not impossible but it would be very difficult, especially for a first time builder.  Besides the epoxy issues, you would have to keep the plywood dry.  Epoxy over wet wood sound like a huge problem to me.  Again, not impossible but you will spend so much time wrapping/unwrapping plus waiting for a good weather window that your build would be glacially slow.  Even more so with "help" from three kids.

If you really want one of those boats and don't have inside space, I would definately go the class route.  When you get home you will still have lots (~50%) of the work left to do so there will be plenty of opurtunity for the kids to help.  Buy the model and have them help you build that so they see what you did in class.  

I built a wooden Sunfish clone in an unheated, unairconditioned detached garage in Annapolis. For the most part I used MAS epoxy with slow hardener. In the cool months I switched to fast hardener. When it became too cold for that (below 45º or so) I found something else to do. Never had a problem with blush or fogging. Your idea to use a portable car port should work fine.

Whichever varnish you select, the manufacturer's data sheet will give you the minimum/maximum temperature and humidity. Knowing those, it's a matter of paying attention to the weather and varnishing when conditions are right. It's doable: there's a lot of varnish on boats in Annapolis that never get hauled inside.

I would not varnish the rail before installation. Bending and clamping the rail will just bugger up the varnish so you'd have to re-varnish it anyway.

You might want to practice varnishing on a scrap piece of plywood before deciding on how much to paint vs. varnish.

I am currently building my NED in a detached garage with no electricity, heat, etc.  It is fine.  I am now putting on the rails.  I found I could not do this in the garage, so I moved it outside for this step, then once the epoxy has "hardened" I engaged the family to help me move it back in the garage (it gets pretty heavy with 30 some clamps on it.

Building outside has been done, check out this video...  https://www.youtube.com/watch?v=ncSjnYlqDZ8  

   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!

   In Annapolis, we get plenty of dry stretches so I'm not sure I'd fret too much about dew and rain, but you do want a cover or canopy to keep the sun off, both your head and the epoxy, and the worst of the rain.  And when it comes to finishing, I always was out in the open when refinishing the brightwork on my big boat, so why not?  Definitely use the slow hardener and get the epoxying done in the warm weather.  When it goes up to 95F in July, knock off early, get some iced tea and shade and wait until the next day!  And the CLC kits are generally so good, they pretty much slide/snap together like jigsaw puzzles (with the directions!) so I can see kids helping just fine.  If one of them likes sanding, sandpaper is pretty safe for everyone, though I'd keep some of those COVID masks around to protect from the dust. And there's all the wires to thread through holes.  And I'd let them pick the color scheme for finishing.  I did that for my nephew years ago for the canoe I built for him.  Bright lobster buoy orange outside, cream inside....yes, it was the "Dreamsicle".  It worked.  With enough masking tape, it doesn't matter if the brushwork is a bit inaccurate.

Those portable tent like temporary shelters work ok at keeping things dry and shaded enough to allow for an outside build in the summer..  However; flies, mosquitoes leaves and other airborn dust, etc will find its way onto your fresh paint or varnish. Usually they can be scraped off without leaving too much evidence. Only you will know the finish is not quite 100% perfect.  Cheers.  

   Thanks everybody for the very sound advice and points of view! I'm gonna pull the trigger. I've been itching to build one for a long time and I know my kids will dig the process no matter how much they're involved in the actual build. Sometimes they just like helping dad read the instructions :)