Great job on the trainer be sure that you don't end up tail-heavy. Our experience is that RC planes are usually much more tail-heavy than c/l'ers and that converting them takes care to make sure that they are flyable. Even more so, since this is a trainer and your plan is to use a pretty light-weight engine. As a starting point, I would make sure that you balance no more than 10% or so (of the chord) and even as far forward as the leading edge.

Good luck to the young lady - let us know how it works out.