EXCLUSIVE: Kay and Larry Woodcock prayed desperately for their grandchildren to be found. And after facing a horrible tragedy, the grieving grandparents are determined to find out what caused their deaths.

In June of this year, police found the remains of 17-year-old Tylee Ryan and her 7-year-old brother Josh "JJ" Vallow at the Idaho property belonging to their mother’s new husband. The case gained national attention for the couple’s doomsday beliefs and the mysterious deaths of their former spouses.

On Sunday, true-crime network Investigation Discovery (ID) is airing a three-hour documentary titled "Doomsday: The Missing Children," which follows the Woodcocks' quest to learn the truth. Cameras follow them as they travel the country to gather information from local law enforcement and concerned friends in hopes of creating a timeline for matriarch Lori Vallow. 

The bizarre case of Lori Vallow is the subject of a new true crime documentary airing on Investigation Discovery (ID) titled 'Doomsday: The Missing Children.'

The bizarre case of Lori Vallow is the subject of a new true crime documentary airing on Investigation Discovery (ID) titled 'Doomsday: The Missing Children.' (ID)

The special will examine Lori’s religious beliefs and the bizarre events that led up to the children’s deaths.

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Attorneys for Lori, 47, and her husband Chad Daybell did not immediately respond to Fox News’ request for comment.

"JJ was a pistol," Larry recalled to Fox News. "His energy ran a hundred miles an hour and he was just so bright. And Tylee was an intelligent young lady who had a bright future ahead of her. It was hard not to love them."

Kay and Larry Woodcock with JJ Vallow.

Kay and Larry Woodcock with JJ Vallow. (ID)

Their mother Lori used to be described as a doting mother who seemingly had a happy marriage to former husband Charles Vallow.

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"I thought it was a great relationship," Kay recalled. "Charles and Lori were the all-American couple. They were active in their church communities and so attentive to their children. Charles was a hard worker and he wanted his family to have a good living. Sure as a couple, you saw them snip a few times at each other over the years, maybe three times at most out of 12 years together. They just seemed so compatible."

But the marriage was beginning to crumble behind closed doors.

Charles Vallow originally claimed that his wife Lori Vallow was becoming obsessed with the end of times.

Charles Vallow originally claimed that his wife Lori Vallow was becoming obsessed with the end of times. (ID)

In February 2019, Charles had filed for divorce, alleging in court documents that he feared Lori will kill him and claimed that she’d developed strange, doomsday-cult-like beliefs, She allegedly began calling herself "a god assigned to carry out the work of the 144,000 at Christ’s second coming in July 2020."

Kay said Charles confided in her about his concerns but admitted she thought the claims were outrageous.

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"I just kept telling him, ‘What are you talking about?’" she recalled. "It must be her hormones. She just needed to see a doctor. But Charles was devastated when they split up. He kept saying, ‘She doesn’t want me anymore.’ Well, I had a hard time digesting that. I just thought she was trying to hurt him with words."

Kay and Larry Woodcock described Charles Vallow (pictured here with JJ) as a doting father.

Kay and Larry Woodcock described Charles Vallow (pictured here with JJ) as a doting father. (ID)

In July 2019, Charles was shot and killed at the family’s suburban Phoenix home by Lori’s brother, Alex Cox. He told police the shooting was in self-defense and that Charles had come at him with a baseball bat. In December of that year, Cox passed away from a pulmonary blood clot.

After her estranged husband’s death, Lori moved from Phoenix to Idaho with the children. She reportedly began spending time with Daybell, an old acquaintance who has written several books focused on doomsday scenarios. 

The Woodcocks said they attempted numerous times to contact the children, but phone calls with JJ, in particular, grew infrequent until they stopped altogether.

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Pals viewed Lori Vallow (pictured here with Tylee and JJ) as a caring mother before her marriage began to crumble.

Pals viewed Lori Vallow (pictured here with Tylee and JJ) as a caring mother before her marriage began to crumble. (ID)

"It was a slap in the face," said Kay. "We didn’t even get a chance to make sense of Charles’ death because we were searching for answers desperately. Charles was supposed to outlive us all. He lived a very healthy lifestyle. His only vice was was M&Ms. He loved those children and he was crazy about Lori. And I couldn’t even pick up the phone and talk to my grandson. The last time I saw him was just a 35-second FaceTime."

"Could we have stopped this? I don’t know," Kay continued. "Could we have gotten JJ on time? We would have given Lori money and more just to have those kids back, I don’t care. You don’t want to be a mother anymore and want to ride off into the sunset? Fine -- we’ll take them. I still think about it every day, what we could have done."

In October 2019, Daybell’s longtime wife, Tammy Daybell, passed away. According to her obituary, the 49-year-old fit librarian died of natural causes, and the family declined an autopsy before she was buried in Utah. About two weeks later, Daybell and Lori married on a Hawaii beach.

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Chad Daybell is being charged with destruction, alteration or concealment of evidence and conspiracy to commit destruction, alteration or concealment of evidence, both felony charges. The remains of Lori Vallow Daybell's two children were found on Chad Daybell's property.

Chad Daybell is being charged with destruction, alteration or concealment of evidence and conspiracy to commit destruction, alteration or concealment of evidence, both felony charges. The remains of Lori Vallow Daybell's two children were found on Chad Daybell's property. (ID)

By late November, police in Rexburg showed up at Lori’s apartment to check in on the children at the grandparents’ request. Rexburg Police Lt. Ron Ball said Vallow told him that JJ was in Arizona with a friend. However, that pal told police JJ hadn’t been to her house for months. 

In January 2020, the children's belongings were found in an abandoned Rexburg storage unit. Police searching Lori’s apartment found medicine prescribed to JJ, who had autism, but it was dated January 2019, records show.

In February 2020, Vallow was arrested in Hawaii on felony child desertion charges, USA Today reported. The outlet reported that the couple had allegedly fled after an official welfare check on the children.

Tylee Ryan was described as 'an intelligent young lady who had a bright future ahead of her.'

Tylee Ryan was described as 'an intelligent young lady who had a bright future ahead of her.' (ID)

After the discovery of the children’s remains on Daybell’s property, he was arrested and charged with two counts of destruction, alteration and concealment of evidence, and two counts of conspiracy to commit destruction, alteration or concealment of evidence, People magazine reported.

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The couple has pleaded not guilty and they have not been charged with the deaths of the children, KTVB.com reported.

"I was numb," said Kay when she saw Lori in the courtroom. "She was just so arrogant, it’s ridiculous… she’s an evil monster."

A trial in the Lori Vallow case has been set for April 2021.

A trial in the Lori Vallow case has been set for April 2021. (ID)

A longtime pal of Lori's has since alleged that the mother described the children as "zombies" -- or those whose mortal spirits have left their bodies and now serve as hosts for dark spirits. In response to the allegation, Kay said, "Everyone was a zombie in Lori's eyes if you weren't on her side."

A trial in the case has been set for April 2021, according to the NBC affiliated TV station in Boise, Idaho, KTVB. Kay is hoping to finally get closure after being faced with so much tragedy.

"I just feel after going through all of this, you never really know someone," she said. "I’m a naive person. I trust people easily. But Lori broke that trust in me."

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Grandparents Larry and Kay Woodcock hope to finally get closure after enduring tragedy.

Grandparents Larry and Kay Woodcock hope to finally get closure after enduring tragedy. (ID)

"Doomsday: The Missing Children" airs Sunday, Dec. 20 at 9 p.m. ET on ID. The Associated Press contributed to this report.