I have had a lot of experience with kids in Cub Scouts and grade school with the AMA Delta Dart, aka AMA cub. It is rubber powered free fight and is built with a safety razor blade and white glue. They can built in one session, especially if the leader makes the dihedral joint with medium CA. They are cheap when purchased in bulk from AMA. They could be used to whet the kids' appetite before doing the R/C trainer.
I have built many planes from kits, plans, and scratch. A kit will likely cost less than a plans built model and it will save the hassle of sourcing everything you need.
Balsa USA has a trainer kit designed to introduce newcomers to building. I have no direct experience with it, and it may be bigger than what you desire. It would not hurt to check it out.
Good luck with your work with the kids!!
Chuck