I think you have been given a hard time OP.
If the child is not old enough/responsible enough/skilled enough to ride the scooter safely then she should have an adult supervising her.
Sounds as if the parents have bought her a gift which is firstly illegal to use on the road or pavement, and secondly have let her use it without any tuition on how to do it safely. They have then let her ride without any form of safety equipment - no helmet, lights etc to protect her. They are the ones most responsible for any injury to their child.
DD had a V scooter at 9. When she first used it, she had a helmet and one of us was supervising - we would go for a walk and she would scoot on ahead to a point and wait for us - we could see her most if not all of the time. However, she knew that if people were walking in the other direction, she needed to stop or slow down and would do that.
She only rode without us once we felt comfortable that she was responsible enough to do so.
We wouldn't have bought her an electric scooter anyway, but in the situation you described i would have blamed her for not being careful enough with how she was riding.
The girl should have only been going at a speed that she could control the scooter. If a younger child/animal had stepped on to the pavement, the girl would not have been able to stop, and things could have been worse. Who would the parent's blame then - the toddler, the animal? Probably not their child or themselves.
The way she got up and checked the scooter before leaving, makes it sound as if she has fallen off before, and falling again didn't faze her.
Not sure what you were supposed to do when you saw her go in one direction, You have already explained that you looked and could not see her coming back due to it being dark and her wearing black, What to people think you should have done differently - climbed over the handbrake and got out of the passenger seat, just in case she scooted back? Sat in the car all evening until there had been no sight of her for 10 minutes (she could have then still returned). Even if you had got out of the passenger seat, you would have needed to step on to the pavement from between cars, so would have still be difficult to see her approaching at speed in the dark.
People are focussing on you not seeing the girl, but are forgetting that if you should have been able to see her, then she should have been able to see you - especially seeing as cars light up when the door is opened.
Hope the girl was ok and there is no come back, but more importantly hope that you are ok. It is frightening when you start thinking what could have happended,