Jörg

One idea no one has suggested, why not 2 boats? At 9 and 14 the kids are certainly old enough to want to do some of the paddling, every one will have more leg room, you'll be able to split up who paddles with whom (even the closest family will grow tired of sitting within a few feet of each other for 8 weeks), and portages will be easier with 2 lighter loads insteaad of one heavy one. Plus it opens up a lot more existing designs, and proven plans will be a cheaper and easier build. The only real down side is you have to build an extra boat, and that really isn't a down side now is it?

If the kids are really into this trip, they should be into helping with the build(s) as well. This sounds like an amazing adventure, just think of how much pride the kids would have if they had a hand in building and piloting their own craft.

Just a thought
Tom