It was a full automatic submachine gun. The exact Uzi model would dictate the rate of fire - it varies a lot with the year and variant. And the clip size can also vary from 20 to 70 rounds in the magazine.
But any of these models can dump the entire magazine in a matter of a second or two, with trigger held.

If you had ever shot a full automatic weapon before, with that high of a rate of fire, you would know that it is a steady push until that trigger is released (or runs out of ammo). That push pivots on your hand from the grip. Sending the weapon not only pushing back but upward with force.
For a 9 year old girl, who had never shot one before and had no idea what to expect - it was inevitable that this would happen.
So why was the instructor not right up behind her, with both his hands, on both her arms, to help steady her?
And I would argue to, why a 9 year old even had that kind of weapon in hand.