You are right, one could write a book on the change up.
I don't think my son uses his CU enough however that is not his bread and butter pitch.
I believe that pitchers have different stuff and all use it differently. My son uses his CU for a big swing and a miss and uses his sinking 2 seam FB for the 3 hopper to the ss.
BTW, son has told me that JBB's son has a great CU and he knows how to use it, for sure!
It is common to use a CU first pitch strike when players are young because young hitters go for the first pitch.
I think it is a good idea to encourage the use of the change for younger pitchers, it is important for them to learn to trust it.