The girl was taken to hospital by air ambulance.
Somehow she was hit by the quad bike. We are investigating what happened, says police spokesperson Ola Österling.
The driver, who is in his 20s, was also injured and has been taken to hospital. According to police, the man is suspected of gross negligence in traffic, causing serious bodily harm, and driving under the influence of drugs.
The accident occurred in Botkyrka municipality on a side road or footpath at Hågelbyleden. The alarm reached the police shortly after 8 p.m. on Sunday.
The quad bike has been seized.
