Jenkins has a history of violence against her children, and is on probation for fracturing the skull of Robert's infant half brother. That boy was taken from her by the state in 2008.
According to court records, Jenkins "did willfully inflict" injury to the infant "by striking the child's head on a surface, causing a fracture to the child's skull," on Oct. 19, 2008. She received a sentenced of 29 days of work release and was put on probation for two years.
The Associated Press reported that Jenkins also has a third child, a 2 1/2-year-old daughter, whose father has custody. Jenkins has visitation rights. Ehrlick is banned from being alone with the girl, according to the AP, but court documents don't say why.
Ehrlick has a criminal record including convictions for burglary, grand theft, battery and possession of drug paraphernalia.
The discovery of Robert's body came after several police agencies spent the weekend digging up the backyard of an Ada County home that police told ABC News affiliate KIVI-TV in Boise is rented by people who know Erhlick.
At the time, police declined to say what was found to lead them to the backyard, saying only that they were following multiple leads and searching multiple locations looking for the boy.
After the search, though, Kerns said that with the help of the FBI they had found evidence they hoped would bring them closer to finding out what happened to the boy.
He also said that evidence discovered during in a search of the Jenkins' home "shows there are suspicious circumstances regarding Robert's disappearance."
KIVI-TV reported that Boise police spent hours at Jenkins' home, towing the family's car and leaving with bags of evidence.
ABCNews.com's Sarah Netter and Emily Friedman contributed to this report.