Police in Yangjiang in the southern province of Guangdong said in a statement that a 29-year-old woman surnamed Chen was being investigated after the video went viral.
She lives with the girl's grandfather, a 50-year-old widower surnamed Ye. Chen told police that she started the scooter to scare the girl, who didn't want to leave home with the rest of the family.
She said that she accidentally ran over the girl's legs, but later checked and found that the girl wasn't seriously injured. The police statement said the girl suffered leg injuries.
The video shows another child dragging the apparently already injured six-year-old away from Chen, when she slowly accelerates and runs her scooter over her legs. The girl's cries are audible.
'Hearing her cry is killing me'
Police said the girl's parents had divorced but when her father found out about the incident, which happened earlier this week, he returned from the city to take care of her.
Authorities said they will decide whether to prosecute Chen and the girl's grandfather after the investigation.
It's not clear who shot the video or who posted it online but Chinese social media was flooded with angry comments, making it a top trending topic on Weibo, a social network similar to Twitter.
When CNN called Yangjiang police, they referred us to the statement, which was posted on its official Weibo social media account on Tuesday. "The child is so young; hearing her cry in the video is killing me," said one online commenter.
Although China has laws that prohibit the abuse of children, none of them define what constitutes child abuse and there is no public body in charge of monitoring child abuse, according to scholars at Beijing Normal University.
China has more than 61 million children that grow up without one or both their parents, who are usually migrant workers. They are thought to be particularly vulnerable to abuse.