The boy said he was uncomfortable with leaving the girl alone because she was asleep, but the woman - Cindy Patriarchias - said the girl would be "fine because they weren't going to be out long," according to a criminal complaint.
Patriarchias and her boyfriend, Edmond Gonzales, left the girl in the cage while they took her three children to an elementary school Friday to watch the movie, Las Cruces police said.
Patriarchias' estranged husband called police when he arrived at the school and learned that the 8-year-old was still at the mobile home.
Officers later discovered a large homemade wooden cage in the corner of a bedroom with the girl inside. The cage had two latches on the door, a baby crib's mattress inside and was about 2 1/2 feet wide and 4 feet tall, police said.
Another child told police that the girl was usually placed in the cage when she misbehaved, the complaint said.
Patriarchias had been trying to adopt the girl but hadn't been able to reach the girl's birth mother to get her consent, police have said. The girl suffers from microcephaly - a condition that leaves people with an abnormally small head.
John Patriarchias, the estranged husband, told police that the cage was built to keep the girl from getting out of her bed at night and to prevent her from getting into cleaning supplies.
Cindy Patriarchias, 33, has been charged with negligently causing child abuse. Gonzales, 37, was charged with negligently permitting child abuse. Dona County Magistrate Court records indicated that they appeared in court Monday but didn't enter pleas.
A Las Cruces Public Schools spokesman told the Las Cruces Sun-News in an email Tuesday that Patriarchias was a part-time lunch monitor and volunteer at Columbia Elementary. The spokesman added that Patriarchias "has been terminated" and is no longer allowed to volunteer at the school.
Gonzales was being held on a $25,000 cash-only bond. Authorities said Patriarchias has been released.
No attorneys are listed for Patriarchias or Gonzales.