[Missed Miss]

Maybe you can start them off slowly with comedic gender benders like "Sorority Boys", "She's The Man", "He's My Girl" and one I just saw for the first time last night, "Dr. Jekyll And Ms. Hyde". While looking for one I saw on YT a few months ago with a very similar title, I think, where these two young people discovered that they have the ability to quickly change gender, but not exactly when they want to, I found a few clips from a pretty cool sci-fi type movie from the early `70's called, "Dr. Jekyll And Sister Hyde". Basically, it's the same as the one I just watched last night, but it's more like the original story, except that, once he drinks the potion, he turns into a woman. Brian Clemens wrote it, so it HAS to be good!
Anyway, just take note of what their reactions are to this type of stuff, then, maybe hit them with the heavy guns of that other movie I mentioned last night. Kinda like boiling frogs in a pot of water. Turn up the heat gradually and they won't notice they're being cooked.