Disclaimer: I am not a pitching coach, merely the dad of a 12U pitcher.
On the advice of my DD's pitching coach, who was a pretty good D1 pitcher and a self proclaimed "physics nerd" I would stay away from your girls using a flip change.
From what I was told, the way the ball spins with the flip change if the batter gets good wood (composite?) on it, their chances of murdering it is better. The rotation spins back the opposite way and the ball will really carry.
DD's pitching coach teaches a circle change, a "shove change up" and something I don't even know the name of depending on what her students seem most physically comfortable with/capable of throwing best.
I personally think the flip change is one of the best pitches in softball. That’s one of the greatest things about this sport and pitching. There are many different ways and thoughts to get batters out. Now I agree the spin is back spin which if properly squared up can help it travel. But the other side is the pitcher is providing no power for the hitter and normally the speed is so different it can really mess with timing of the batter. It does take a good coach to use the flip as it is a situational pitch. If it is being called for strikes it is great to get a cheap strike on the batters. Since most hitters are taught to lay off it with less then two strikes. You will also get a bunch of weak pop ups with it. To me it’s best to use it early and often in the counts. If teams start sitting on fastball waiting for it then throw it outside off the plate to chase. Also use it with a long at bat. If batter keeps fouling off pitches it can get a easy pop up or throw off timing for the next pitch. It is one of the hardest pitches to master but well worth the time in the long run.