As a ex Little League, Pony, American Legion, Coach, I totally agree with your take on kids not learning the proper fundamentals. That is: No curveballs, proper wind up form (fundamentals) to strengthen legs, arms & backs). Form is crucial, to not wrap the ball, extend the arm behind, open up the shoulders to a degree, then the hamstrings & legs maximize the speed of a pitch, without blowing a young arm out. Curveballs are out of the question. I went to school with Jim Slaton, ex Brewers, Tigers pitcher. I worked with Gary Peter's Brother, (1963 AL ROY, 1964 AL Picher of the year). Both emphasized not throwing a curveball, like Jim threw his 1st as a Senior in High School. Peters relied on his slider when he joined the Minor Lg. White Sox.
A great pitcher often looks like a stork forming a tee, when he delivers the ball, downsizing the distance between the ball coming out of the hand to the plate. Nothing like Randy Johnson eliminating 12 feet of distance between his nasty pitches at 100 mph.
Kids need to first learn control, followed by pinpoint accuracy to hit the in-out, up-down locations. Off Speed pitches are a trump 3rd strike out. (if thrown right). Merely moving the ball around, will net most outs. Curveballs only wreck elbows at young ages, so do sliders, but boy are they fun to throw when one grows older. A good slider has eyes in it, and ducks when a hitter swings!