Brittany Gosney, 29, left her son, James Robert Hutchinson, 6, at a rural park and then dragged his body for a distance as the boy desperately clung to the car before returning to dump his body in the Ohio River, police say.
The girl is believed to have had long dark hair and been between 6.5 and 10 years old when she died.
Officials say the children were aged 13 to 18 and had all been deemed as endangered.
The U.S. Marshals Service said it will continue searching for missing children in northern Ohio over the next few weeks.
"There is no more meaningful work that law enforcement does than rescuing children," U.S. Marshals Service Director Donald Washington said while announcing that 39 missing children had been found after a two-week operation.
Daybell and his wife, Lori Vallow, have been the subject on an ongoing investigation into Vallow's missing children, 17-year-old Tylee Ryan and 8-year-old J.J. Vallow. The two children have not been seen since September 2019.
Daybell was taken into police custody on Tuesday and local authorities confirmed the discovery of human remains at his Idaho home. Vallow has been in jail since February 20 when she failed to present 17-year-old Tylee and 7-year-old J.J. to police following a court order.
Mark Degner and Bryan Hayes have not been seen since they ran away from school in 2005.
Joshua Vallow and his older sister Tylee Ryan were last seen in Rexburg, Idaho, in September.
Local police declared the teenager missing on Wednesday night and managed to reunite him with his parents less than 24 hours later.
The body of missing Florida boy, Jordan Belliveau, has been found.
"This is a classic example of the adage 'No good deed goes unpunished,'" Health and Human Services Deputy Secretary Eric Hargan said.
"I think the idea that they're 'lost' is hyperbole to try and create an issue where I don't really think there is one," Santorum said. "Other than the fact that the bureaucracy—surprise, surprise—doesn't work very well."
Jody Haskin still on the run from police as suspect believed to have links to North Dakota.
Wang Mingqing and Liu Chengying lost their 3-year-old daughter in 1994.
Multiple law enforcement agencies are looking for Adriana Coronado, who reportedly was abducted early Saturday in Katy, Texas.
A family acquaintance has been charged with kidnapping the teenage sisters.
A prime example: The case of Sage and Isaac Cook, two brothers whom law enforcement recently found in Mexico.
At least 340 children disappeared in the U.K. between January and September of this year.