See to me, calling 6 fastballs in a row is not developing a pitcher. Regardless of the count, what the score is, or how behind the batter was on the one strike in the count. I guess that's the major difference of opinion we have here. By your scenario if a girl has walked 8 batters in 4 innings throwing all fastballs, you should keep calling fastballs to develop her. My theory is different. Work on locating your fastball in the backyard. Come game day, we're going to mix things up a bit and not care if a girl gets a single to right field in a meaningless game in November. Make things more like they will be she matures and moves up in age bracket.Since we are all about development ( you are the one who made the initial argument-starting comment about this) a 12U pitcher learning how to get a fastball over the plate for a strike when she needs to is probably pretty high up there no? A pitcher who can only get her CU over the plate isn't going to last very long in 12U nvm at the higher levels.
So yes throwing a fastball to a hitter who is really late on the FB is a good idea in my book at the 12U level both from a developmental perspective and a strategic perspective (she did get a hit in that AB right....? )