With all appreciation for what you're trying to do for these kids, I gotta say I think you're setting them up for failure.

With inexperienced designers building from scratch, the likelihood of every plane crashing on takeoff or shortly therafter is high. Give them a project that they can fail at a few times without destroying their work, so that their chances of success is high if they do the work. I'd go with rubber band free flight planes. You can do competition in landing accuracy, duration, and altitude. And they can get the setup right on them with partial motors without much chance of damage. The cost will be cheaper, and you can teach most of the same engineering principles.