Christopher Carlson, 45, is accused of beating the boys, ages 12, 9 and 8, and denying them food and water.

Federal authorities said Carlson abused the children in hikes up and down the canyon, restricting food and water to the point that the kids were vomiting and walking on ulcerated blisters.

All three children suffered various injuries, authorities said.

The U.S. attorney's office in Arizona said rangers and a passer-by took note of the condition of the boys and contacted authorities.

According to a probable cause affidavit, one of the hikes took place last weekend and covered 19 miles as temperatures reached 108 degrees. The boys told authorities that they were abused over the course of several weeks in Arizona and Central America.

National Park Service employees told federal investigators that they first became aware of Carlson and the boys on Aug. 15.

Parks workers said they confronted Carlson and asked the boys if they would like food or water, but that Carlson declined and said they were fine.

Parks workers said they later witnessed Carlson abusing the boys.

The boys told authorities that they were "hit, pushed, choked, kicked, pinched, squeezed and whipped," according to court documents.

One of the boys detailed an allegation that his face was slammed into a rock because he wasn't walking fast enough to suit his grandfather.

"Carlson pushed him down onto a rock, kicked him and smacked his face down onto the rock," the probable cause affidavit read. "As a result, his leg was cut, lips were bleeding and his nose was scratched … Carson told him to wipe his lips so the ranger wouldn't see the blood."

Carlson appeared in court in Arizona on Thursday, showing no emotion, reporters said.

A park ranger testified that Carlson told him that the kids were overweight and needed to “toughen up.” He said his goal was to prepare them for the cruel world and to strengthen them during recent hikes in tropical locales, including Honduras, Belize and Guatemala, the ranger said.

"This was a trip of a lifetime. He offered to take them to Central America and also on this big road trip out to Disneyland and back," said Mik Jordahl, an attorney for the boys' mother, Tara Dahaher. "I think any parent would think, hey, that's great. Unfortunately, it's turned into a trip of a nightmare, really."

Dahaher wept openly through most of the hearing.

"I just want to say I love my kids, and I’m here with them," she said outside court.

The ranger also testified that children’s familiarity with types of marijuana and different strains was quite abnormal.

Carlson lives on Indianapolis' northeast side. Neighbors were shocked by the allegations.

"He was a good neighbor. He took care of his yard, his dogs. I would have never thought that," said Michelle Roach.

The children are in protective custody.