A 9-year-old boy testified Wednesday that his grandfather hit him in the face, causing his nose to bleed, and kicked him and his two brothers during a pair of miles-long hikes at the Grand Canyon last summer.

Prosecutors have alleged that Christopher Alan Carlson deprived his three grandsons of food and water and pushed, choked and repeatedly kicked them during hikes on Aug. 15 and Aug. 28, when the temperature soared to 108 degrees.

The statements from the boy in the trial were the first he's made publicly about the charges against his grandfather, who faces six counts of felony child abuse. The 45-year-old from Indianapolis has pleaded not guilty in the case and could face life in prison. The trial opened in Phoenix federal court on Wednesday.

Under questioning from prosecutor Camille Bibles, the boy also said that Carlson would pinch pressure points in their back that "kinda hurt" during the hike up from the Grand Canyon.

"We would slow down because we were kind of exhausted. He started kicking us and telling us we had to hurry," he said, adding that his eldest brother "kept falling and he had cramps and his stomach started hurting on the way out."

A criminal complaint said Carlson put his grandsons -- who were 12, 9 and 8 years old at the time -- in circumstances "likely to cause death or serious bodily injuries."

But investigators have also said that Carlson told them that the boys were overweight and that he thought hiking the Grand Canyon would help get them into shape.

Defense attorney Jeffrey Williams portrayed Carlson as an active health nut who had a firm hand and wanted to show the boys the world. Like anyone after a long hike, the boys were tired, hungry and thirsty, but Carlson only allowed the boys to eat healthy food like tofu, hummus and veggie burgers, Williams said in his opening statement.

"I suppose to an 8, 9 or 10-year-old that might seem like child abuse if you like cheeseburgers, French fries and pizza," he said. "He wanted to get them from behind the TV, the games and fast food."

The 9-year-old, the youngest of the three brothers, testified that he experienced cramping, nausea and hunger during the miles-long treks.

But he also told jurors that his grandfather took the boys on many "awesome" adventurous trips. In between the two hikes, Carlson took the boys on a tour of the Hoover Dam, to rides atop the Stratosphere hotel and a Criss Angel magic show in Las Vegas, and to Disneyland in California.

Jurors smiled as the boy also spoke of trips to Mexico, Belize, Honduras and across the western United States last summer with Carlson and his two older brothers. The trips often included long hikes, swimming and fishing in the ocean and thrill rides at amusement parks.

Although investigators have said Carlson withheld food and water, the boy testified that he and his brothers were allowed to drink water most of the time and snacked on celery, carrots, tofu and low-carb hummus during the hikes.

The criminal complaint said that one brother feared that another brother would fall to his death because Carlson forced him to walk on the edge of the trail even though he was cramping and falling down. Another brother said that when he fell, Carlson picked him up by the throat and threw him to the ground, and another said Carlson had slammed his face into a rock, the complaint said.

Investigators said the boys were covered in cuts, bruises and scars that backed up their stories. But Williams told jurors Wednesday the injuries "are consistent with boys who are just normal boys" who had an especially active summer.

"There's nothing life-threatening about a blister," Williams said. "If these kids were abused they were abused by kindness and all the fun and trips they took over the summer."

A ranger with binoculars spotted the group during their 19-mile hike on Aug. 28, the same day a man died on another trail from heat exposure. The ranger reported seeing Carlson shoving the oldest boy and whipping him with a rolled-up T-shirt.

Rangers fed the boys and gave them water after one showed symptoms of heat stroke and the other two had signs of heat exhaustion and dehydration.

Under questioning from Williams, the boy said the kicking didn't hurt much most of the time and that he often gets nosebleeds for no reason. He also said his grandfather was in a hurry on the Aug. 28 hike to get to the top of the Grand Canyon so they could watch the sunset together.

But Bibles said in her opening statement that Carlson used the Grand Canyon as a weapon in child abuse.

"These hikes became a life or death situation for these children at the hands of the person that was supposed to be protecting them," she said.