A Florida family is in mourning after police say a 12-year-old girl got behind the wheel of a vehicle before hitting and killing her two-year-old brother.

The tragic crash happened outside a residence in south Orlando at about 6.25pm local time (11.25pm BST) on Wednesday (10 May).

The young girl started the ignition of the 2015 Chevrolet Traverse SUV as the family were preparing to leave their home in Trafalgar Drive, according to an incident report seen by local news channel WFTV9.

The SUV was somehow put into reverse and rolled backward, hitting the girl's toddler brother who was standing in the driveway.

The injured boy was taken to Nemours Children's Hospital in the car that hit him and then transferred to Osceola Regional Medical Center where he later died.

It was not clear why the girl was behind the wheel of the car or how it came to be put in reverse.

Florida Highway Patrol (FHP) called it a "tragic fatality that pulls at the heart". They said they don't expect to file any charges.

"Our hearts and our thoughts and prayers go out to the family," Lt Mark CastleBerry, of the FHP, told WFTV9.

"Something like this is tragic and we can't begin to express words that would help ease that pain. The best we can do is give them closure and do a proper investigation."

No further details of the incident have been released, not have the identities of the family and children. Neighbour Nicholas Catanzaro, 19, was arriving home from the gym after the incident when he saw a child crying in the driveway near some blood. He found out what happened a short while later. "They're such a nice family. I feel so bad that this happened to them," he told the Orlando Sentinel.

Catanzaro said the family involved had five children, two of whom are high-school aged. "I didn't really think it was anything serious," he said. "If I had gotten home a minute of two earlier, I probably would have seen it happen."