Christopher Carlson, of Indianapolis, remained jailed Thursday on six counts of child abuse. The boys, ages 12, 9 and 8, told investigators that they had been hit, pushed, choked, pinched and squeezed during trips into the canyon from the South Rim last month.
A ranger with binoculars spotted the group on its most recent hike -- a trek last weekend on the popular Bright Angel Trail and saw Carlson shoving the oldest boy and whipping him with a rolled-up T-shirt, authorities said.
Rangers fed and hydrated the boys and they were placed in the care of child protective services. One boy had symptoms of heat stroke, while the other two exhibited signs of heat exhaustion and dehydration.
National Park Service Special Agent Chris Smith testified that Carlson told authorities that the boys had been overweight and that he thought the hike would get them into shape.
"He told me that he loved his grandchildren very much, but at the same time there were tough people in the world and his grandchildren needed to be tough as well," Smith said.
Authorities said Carlson tortured and beat the boys, and instructed them to lie to park rangers about any injuries. Rangers and passers-by noted the alleged abuse by Carlson, according to court documents.
The boys said Carlson also forced their fingers down their throats, making them vomit.
The Bright Angel Trail can be deceiving. It starts at the top of the canyon at around 7,000 feet in elevation and drops to 2,400 feet by the river, and the temperature varies widely. The Park Service advises hikers not to make the trip to the river and back in one day. Warning signs are posted at the trailhead and along the trail.
Carlson took the children on the latest hike on Sunday when the temperature hit 108 degrees at Phantom Ranch by the Colorado River. A man died the same day on another trail at the Grand Canyon due to heat exposure.
Defense attorney Luke Mulligan questioned the children's statements, saying it seemed improbable that they could have completed the hike without food and water. He also said the rangers could have removed the children from the canyon had they believed the children were at risk of serious injury or death.
"If the rangers didn't perceive it, are we going to put an extra burden on my client to perceive it?" Mulligan said.
But prosecutors said a ranger was successful in preventing Carlson from reaching the river on an earlier hike on Aug. 15, cooling down the children and giving a heads-up to other authorities in the park. Camille Bibles said Carlson was intent on avoiding rangers during the second trip.
"We're looking at the defendant's actions here, not putting the rangers on trial," she said.
A federal magistrate found probable cause for allegations of child abuse and determined that Carlson, 45, was a flight risk and a danger to children.
The boy's mother, Tara Danaher, of Indianapolis, sobbed at a court hearing Thursday. She said her children went on trips with their grandfather this summer, including to Central America and Jamaica, where they have family friends. The highlight of the latest trip that included the Grand Canyon was supposed to be Disneyland, she said.
Danaher, 28, said she talked with her children throughout the summer and that they never expressed any concerns.
"I don't want to say I can't believe it because anything is possible in this world," she said during breaks in the hearing. "I want to know what the hell happened."
August is the busiest month for search and rescue operations, with heat being the leading contributing factor, according to the Park Service. Of the 286 rescue operations last year, 75 percent of them occurred on the Bright Angel Trail.