Boise, Idaho — The mother of a child and a teen missing since September failed to meet a court-ordered deadline to bring them to authorities in Rexburg, Idaho on Thursday. That clears the way for a judge to potentially hold Lori Vallow in contempt of court, a legal move that could allow prosecutors to seek to have her extradited from Hawaii to Idaho to face charges.

Seven-year-old Joshua "JJ" Vallow and 17-year-old Tylee Ryan haven't been seen since late September, and police in Rexburg, Idaho, have said they "strongly believe that Joshua and Tylee's lives are in danger."

Lori Vallow and Chad Daybell and Joshua "JJ" Vallow and Tylee Ryan. KUTV

"The only word coming to mind right now is 'monster,"' Kay Woodcock, JJ's grandmother, said during a news conference Thursday evening. "All this has just been very disheartening. ... I'm a lot less optimistic at the moment."

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She described Vallow to CBS Boise affiliate KBOI-TV as smug and arrogant. She said she's even considered filing for custody of the children.

Woodcock and her husband, Larry, are pleading for JJ's safe return and that of his sister, CBS News correspondent Nikki Battiste reports.

"Bring the kids, that's it," Kay Woodcock said. "I don't care to ask her anything. I don't care."

"Show me they're alive," Larry added. "The only thing that I care about is those two grandkids. We're here, I'll get on my knees, and I'll beg. Just give me JJ. That's all I want."

Police have said Lori Vallow and her new husband, Chad Daybell, have lied about the children's whereabouts and even their very existence, with Chad Daybell allegedly telling one person that Lori Vallow had no kids, and Lori Vallow allegedly telling another person that her daughter had died more than a year earlier.

The tangled case spans multiple states and includes investigations into three separate deaths: Lori Vallow's estranged husband, Charles Vallow, was shot and killed in Phoenix in July by Lori's brother, Alex Cox. Cox, who claimed the shooting was in self-defense, died of unknown causes in December.

In August, Lori Vallow moved her family to Idaho. In October, Chad Daybell's wife, Tammy Daybell, died of what her obituary said was natural causes. But when Chad Daybell married Lori Vallow just two weeks after Tammy's death, law enforcement became suspicious and ultimately had Tammy Daybell's remains exhumed.

The test results on Tammy Daybell's remains and on those of Alex Cox haven't been released.

The case also involves rumors of a cult: Lori Vallow reportedly believes she is "a god assigned to carry out the work of the 144,000 at Christ's second coming in July 2020," according to divorce documents filed by Charles Vallow. Chad Daybell has written several apocalyptic novels based loosely on Mormon religious theology. Both were involved in a group that promotes preparedness for the biblical end-times.

Police questioned Daybell and Vallow about the missing kids in late November and the couple left town before police returned the next day. On Saturday, they were stopped by investigators in Hawaii, served with the order to produce the kids, and then searched.

Vallow and Daybell are still in Hawaii, Kay Woodcock said.

"I was informed that they're still probably sunning on the beach somewhere while we're sitting here wondering where the kids are," she said.

A contempt of court charge is generally a misdemeanor under Idaho law, with a penalty of just five days in jail and a $5,000 fine. But Woodcock said authorities told her it could be used to seek Lori Vallow's extradition.

"We now know that there's some recourse that can happen after this," she said. "They're going to make her accountable to law enforcement."

It could be some time, however, before the public knows exactly what's going to happen next in the case. Child protection cases in Idaho are sealed, and the Madison County prosecutor had to get special permission from a judge just to reveal that Vallow had been legally ordered to produce her children.

The Woodcocks have offered a $20,000 reward for information leading to the children's discovery.