Lala: I used to believe as you do- deliberate cheating by juniors was rare. Now I think dfferently because the evidence is overwhelming. A significant percentage of the top juniors deliberately cheat. I do not have sour grapes and I do not have a horse in this race. Of course, you have to teach your youngster that life isn't fair and all that jazz.
I've played adult and senior USTA since it started. There is a bit of cheating, but it is about 2-3% of what it is at the junior level. Adults sometmes allow the opponent to make the call when they don't have a good look- I've yet to see that in a junior match. Unfortunately cheating is part of the culture of American junior tennis.