The kids knew him as Mr Bob.

He was there to greet them when they went to school in the morning and when they left in the afternoon, standing in the middle of Leavenworth Road with a stop sign in his hand and a supply of Russell Stover peppermints in his pocket.

For Valentine's Day last week, the kids brought the 88-year-old crossing guard mugs full of candy and made him a card, and for Christmas one year he gave some of them presents.

But on Tuesday morning, the children at Christ the King Catholic School in Kansas City, Kansas, saw Bob Nill for the last time.

It was about five minutes before 8am, five minutes until the first bell rang and Nill's job ushering kids through the crosswalk would be over for the morning. Two young boys, in third and fifth grade, had just stepped off the curb.

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But just then, Nill motioned for them to step back, school principal Cathy Fithian said. He saw a black sedan speeding toward them. He could sense that it wasn't stopping or slowing, despite Nill's big red sign and the school zone's flashing yellow lights.

Nill held his position in the middle of the road, which is where the car struck and killed him.

The two boys came running into Ms Fithian's office in tears, screaming for Mr Bob. The principal consoled them and then went outside to find an awful scene as first responders swarmed the intersection, she told The Washington Post on Tuesday night.

More than 20ft from the crosswalk, she said, she found Nill's stop sign.

“I know he had it in his hand,” she said. “It was right in the center of the road.”

The community is mourning the beloved crossing guard while crediting him with saving the lives of the two boys who had stepped onto the road just as the vehicle approached. He had worked as a crossing guard since 2015, Ms Fithian said.

Kansas City mayor David Alvey thanked Mr Nill “on behalf of our residents for his selfless sacrifice in protecting our children”. Christ the King told parents to “know that Mr Nill is a hero”.

Police spokesman Jonathon Westbrook told The Washington Post that while the official cause of the fatal crash has not been determined, officers do not believe the driver was intoxicated. Distracted driving is the more likely cause, he said.

He could not confirm whether the driver, who remained at the scene, was speeding. But Ms Fithian said that numerous witnesses believed the car was traveling well over the posted 20mph speed limit in the school zone.

As the school considered how to memorialise Nill, “somebody asked me, what would Mr. Nill want?” Ms Fithian said.

“He obviously dedicated his life for kids to be safe,” she said, “and he would want people to protect our children. If we can't protect our children, then what are we doing?”

Bob Nill was born in 1931 and spent nearly his entire life in Kansas, one of his three sons, Bart Nill, told The Post. He served four years in the US Coast Guard until 1951 and worked as a correspondent banker for Commercial National Bank for much of his career, traveling around the state helping smaller banks with business loans and trust and investment services.

But Bart Nill said his father didn't know what to do with his time once he retired about a dozen years ago, from a final stint with the Wyandotte County Appraiser's Office.

His wife, Bart's stepmother, had recently died, leaving him feeling lonely. He would stay up all night watching old John Wayne westerns, Bart Nill said, and would sleep till the afternoon. His sons would come over trying to wake him up, urging him to go back to a normal schedule.

Eventually, Mr Nill agreed.

He saw a job posting for a crossing guard and, to his sons' delight, signed up.

“It was the neatest thing in the world,” Bart Nill said. “It got him back on a schedule again, he got to sleep when he was supposed to - but it also allowed him to spend time with children again, because Dad, he just loved kids. You could sense that.”

Bart Nill said his father started to love working with kids after co-founding the Wyandotte County Sports Association in the 1960s, where he coached youth football. Bob Nill, a former college football player at Baker University in Baldwin City, Kansas, would hear from his former athletes years later, thanking him for what he taught them, his son said.

“While he was certainly a sports nut,” Bart Nill said, “he cared about character. He cared about how people would grow up and take responsibility for themselves and do things the right way. I think these kids picked that up. We all did.”

Bart Nill saw the same attributes in his dad while he was working in the crosswalk. He realised how much his father loved the job when, a few years ago, he worried about taking a single day off work to go to the hospital for a heart procedure. It was the day before the kids went on winter break, Bart Nill said.

“He was so worried about these kids crossing the street that he sent me over,” Bart Nill said. “I was the replacement.”

Perpetually wearing a Kansas City Chiefs stocking cap, Bob Nill waved to every child and parent on foot or in a passing car headed toward Christ the King school, Ms Fithian said. Everyone in the neighbourhood came to know him, she said, and now everyone wants to find a way to memorialise him. The kids suggested a fundraiser in his name in the coming days or weeks, she said.

But in the immediate aftermath of the tragedy, all they could think to do was pray.

Right after Mr Bob was whisked away in an ambulance, the entire school, all 260 students and teachers, stopped to pray the rosary (a Catholic prayer pattern) for the man who kept them safe.

“How many people can say at the moment of their death that hundreds of people are praying for them?” said the principal. “We were offering up those prayers for him at the time he was going to see the Lord.”