Joshua Vallow, 7, and Tylee Ryan, 17, were last seen in September, according to the Rexburg Police Department in Idaho.
On Sunday, the Kaua‘i Police Department in Hawaii said it was assisting the Rexburg Police Department in their investigation. The department said in a statement that it had "served Vallow with an order of petition in an attempt to have her physically produce her children to the Idaho Department of Health and Welfare."
Vallow's attorney, Sean Bartholick, confirmed to NBC News that she had been served the order.
The statement said Vallow and Daybell had been "staying on Kaua‘i for an unconfirmed period of time," and a search warrant had been served on them and their vehicle.
“KPD’s desire is to continue to assist the Rexburg police in whatever way we can with their efforts to locate the missing children,” Kaua‘i Police Chief Todd G. Raybuck said. “It’s our sincere hope that the children have a safe return.”
The department's statement said that the couple does not face charges or arrest, but the Rexburg Police Department said in a statement that if Vallow fails to produce the children within five days, she could be held in contempt of court.
Joshua, known as JJ, and Tylee were not found with Vallow and Daybell in Hawaii, the statement said.
The search for the children began Nov. 26, when police attempted to conduct a welfare check on Joshua, who is adopted and has special needs.
Rexburg police said Vallow and Daybell said Joshua was staying with a family friend in Arizona, but investigators later learned that was a lie. A witness also told police that Vallow had falsely said that Tylee was dead.
Rexburg police said they have learned that Vallow either knows the location of Joshua and Tylee or knows what happened to them.
Police have also said that they believe the children's disappearance could be tied to the investigation into the death of Daybell's first wife, Tammy Daybell, 49. She was found dead in her home in October, and Chad Daybell married Vallow weeks later.
Tammy Daybell's death was initially ruled as natural but has since been classified as suspicious, and her remains were exhumed so an autopsy could be conducted.
Lori Vallow's previous husband, Charles Vallow, also died in July. He was shot to death by Vallow's brother, Alex Cox, during a confrontation.
Lori Vallow and Cox were questioned by police but not charged. Cox died in December, The Associated Press reported.
Before Charles Vallow died, he filed for divorce from Lori Vallow, claiming in divorce documents that his wife believed she was reincarnated and was a god sent to lead people during the second coming of Christ in July 2020, and told her husband that if he got in his way she would kill him.
An attorney who represented him during the divorce proceedings said in a statement that Charles Vallow had a "genuine fear for his life."
And Lori Vallow's older son, Colby Ryan, in a nearly seven-minute YouTube video titled "Mom," pleaded for her to reveal where the kids are.
"I really really want to see Tylee and JJ. I really want this to be over," Ryan said. "I want you to end this."
CORRECTION: (Monday, Jan. 27, 2020, 8:39 p.m. ET) An earlier version of this article included a photo of Tylee’s older brother, Colby Ryan, with his wife, Kelsee, and incorrectly identified them as Vallow and her husband, Daybell. The image has been removed.