Sounds like what you need is a tapered wall
I think that the problem is that you start getting some complex angles when you combine even small amounts of wing twist and dihedral (or polyhedral). You may want to temporarily bond the wing panels together as designed then check for twist by sighting down the wing or some other method. There is one method in particular that comes to mind, but I am having a difficult time finding the link for it right now. If I find it, I will post it. In any case, if the wing appears twisted, you can fix that with the covering, and that may pull the trailing edge back in line.
Good luck! I hope your son enjoys his first plane!