It is so rare that I would love to see how where these girls land in the next several years and hopefully they will keep with it.
A 12YOA or 10YOA kid throwing 50 doesn't prove much other than someone works with the child. Any reasonably athletic kid can throw 50MPH. Since their bodies haven't matured, they can have terrible mechanics and throw reasonably well.
I am impressed by 14YOA throwing 60MPH...and those kids are very rare.
The first separation between "good" pitchers and everyone else is at 60MPH.