I can't offer any suggestions about body disciplines in the youth game, but I can offer my observations while watching youth baseball and, simply out of curiosity, asking a youngster later on, why he did this-or-that.
Changes in arm action can possibly be the result of the surface that the youngster was pitching off of.
I saw this more than just a few times, this summer and last. The same youngster(s) with different styles - or adjustments, all because of the surface that they were working off of.
In an 15U game, I saw the same youngster in the first two innings go from a usual 3/4 arm slot, to a semi-sling shot delivery. A mental note and later I approached the young man with his parents walking back to their SUV, I asked ... why?
The response was... " it was the only what I could get the ball across. My feet kept on diving into holes."
On the other hand, I did get.. " I can throw faster," was another popular response.
Don't take my observations here as gospel, or any close to it with respect to youth pitching form.