A witness told the Dallas Morning News the father was on the sidewalk.
"He was right where he was supposed to be," Alicia Hopkins said. "He wasn't doing anything wrong."
The boy, whose identity had not been released Thursday afternoon, was pronounced dead at the scene. His father and the two drivers were injured.
Witnesses said the chain-reaction began when a car ran a red light, KDFW-TV reported. It hit the pick-up truck, which then mounted the sidewalk and struck the father and son.
They told the TV station that the father pushed his son's stroller on the same route every morning.