Dave,
I'm sure you've worked with catchers that young that had plenty of arm, but reality is that a kid that's in private lessons at 8 yrs old isn't very representative of the overall population. Looking at this specific kid, in this specific video based on the ball dropping in from a pretty high arc, I'm guessing if she makes that throw from 3 ft behind the plate the ball might roll and stop short of 2B. Honestly she looks pretty strong to me for a 10U catcher there are plenty that can't make that throw.
It probably makes sense to teach them proper footwork right away even if they can't be successful, but to me a crow hop or taking a step into the throw is acceptable at that age and not that hard to remove when they get older.
First off I agree this young lady seems to be on the high side of arm strengh for a 10U. Not exactly sure what your reference to 8yr old's in private lessons was about. The only time I see 8yr olds is in my youth age group clinics.
This statement of yours was the gist of my whole thought on this thread.
It probably makes sense to teach them proper footwork right away even if they can't be successful
I see so many catchers from all over the country that no one has ever gone back to and taught them to get rid of that step. I'd like to see them never use it at all.