Stephen M. Howells II, 39, also pleaded guilty to counts of kidnapping and sexually exploiting four other children on 16 occasions between December 2012 and August 2013, according to U.S. Attorney Richard Hartunian of the Northern District of New York.
As part of the plea, Howells admitted that he conspired to kidnap the victims to produce child pornography with his former girlfriend, Nicole Vaisey, who will be tried in June.
Howells sedated the victims, who ranged from age 5 to 11, with drugs that he obtained while working as a registered nurse. He then sexually abused and filmed them, sometimes switching roles with Vaisey.
"The criminal conduct committed by Stephen Howells is beyond comprehension," Hartunian said in a statement.
When Howells was arrested in August, investigators found five computer hard drives containing numerous image and video files of young children engaged in sexually explicit conduct, according to the U.S. Attorney's office.
"Our hearts go out to all the young victims who endured his unspeakable acts. We will continue to do everything in our power to see that he will never abuse another innocent child," Hartunian said.
In August, authorities said, Howells and Vaisey kidnapped two sisters, ages 6 and 12, from a vegetable stand outside their family farm. They lured the girls with a puppy, asking if they wanted to pet the dog, a Doberman pinscher-golden retriever mix named Kaleidoscope, police said. When the older sister got suspicious, Howells pushed the girls into the back of the car.
The girls were held captive for nearly 24 hours, and they were handcuffed together, shackled to a bed and sexually abused, authorities said.
After wide media coverage, a large police response in the area and an Amber Alert, the couple began to worry and decided it was too risky to keep the girls.
Howells abandoned the girls in an isolated area 12 miles from where they were being held, police said.
"The crimes committed by Howells are some of the most horrific imaginable," Andrew W. Vale, FBI special agent in charge of the Albany division, said in a statement. "He carefully planned and carried out these abuses and exploitations."
Howells also pleaded guilty to conspiracy to produce child pornography, producing child pornography and possession of child pornography. A conviction carries a minimum of 15 to 30 years for each count. Howells also would be required to register as a sex offender.
Howells' attorney, Amy Dona, was not immediately available for comment.