It is admirable of you to take your time to train others. Hopefully the malfunction issues doesn't spoil the shooting sport for this young couple. Shooting can be a lot of fun, especially with friends. Get a little friendly competition going. Try each other's guns.
I've never owned a Glock, and probably never will. But I'm curious, is it possible the guns needed to be cleaned before first use? I've experienced this with a few guns. I forgot to clean before first use, the Sig P322 I bought last year. It malfunctioned at least once every magazine, until I took it home and thoroughly cleaned the gun and magazines.