Police in the Phoenix suburb of Mesa responded to the accident on Friday afternoon and said the girl was taken to a local hospital where she later died.
There was no sign that Heap was impaired at the time of the accident.
Heap, 37, played 10 seasons with the Ravens after being selected in the first round of the 2001 NFL Draft. He played two years for Arizona before an injury forced his retirement during the 2012 season.
"We cannot imagine the heartbreak and sorrow Todd and Ashley's family feels right now," the Ravens said in a statement on Saturday.
The Cardinals also offered condolences.
"It is a grief that is beyond words and one which no family should ever experience," the team said in a statement.