He said the Hernandez's parents were on their way to the hospital when police and fire crews intercepted them near 7th Avenue and Broadway Road to provide critical care.
The victim's grandmother, Hilda Quinonez, said it took Xavier only seconds to run from his parent's driveway.
"I think he was just running, thinking we're chasing him to grab him so he was just running all happy because he never goes out of the gate; he never comes outside," Quinonez said through tears.
She said her daughter and son-and-law are good parents, but they're struggling to keep themselves together for their other two sons.
She called the incident a "tragic accident" and is offering a warning to other parents.
"Keep an eye on your kids, always," Quinonez said. "Something can happen in the blink of an eye, but you don't ever think it can happen to you.
The little boy's aunt tells ABC15 their family had gathered for dinner, the adults were cooking and several kids were playing but they don't know how the 1-year-old slipped out of sight.
She says the boy's parents are a young couple who are in total shock. She says they've set up a GoFundMe page to help them pay for funeral costs.
Police say they understand the parents were distraught, but between rush hour traffic and construction in the area, playing catch up did take a few extra minutes and to remind the public that first responders will get to an accident site as fast as they can.
"When you look at the human aspect of it, your child is gravely injured, there's a lot of things racing," said Lt. Mark Tovar Sr. "It did hamper our efforts to help them a little bit."
Yanei and Fatima Garcia live in the mobile home park and heard screams that didn't match the typical yelling of kids playing outside.
"Screams, like pain screams, like really bad," said Yanei who remembers seeing the little boy in his mother's arms as she cried.
"I have it in my head, how I saw him without any life," Yanei said.
The family is holding a car wash Saturday, Dec. 3 near 24th Avenue and Black Canyon Highway in Xavier's honor.