Hal?
The only "trick pitch" I've shown kids is releasing straight from the back swing as the arm starts the circle. I let them know it's illegal and that any ump should call IP if they try it in a game. (I've also shown the behind-the-back pitches, but not for them to try.)
Pitching sling shot is NOT ILLEGAL. It only becomes illegal if the pitcher continues to rotate the arm after the release of the pitch.