What is "baby jail" in the novel, Allegedly?

Mary spent some six years in an adult prison she calls “baby jail.” Because she could not be housed generally with either adult or teen offenders, she spent the majority of that time in isolation. She related a few scenes during those years, including sitting in the library, chained to a chair, when she began her period for the first time. She also recalls a guard who often masturbated over her. Some guards and other inmates felt her situation was unfair, but those people accepted that the justice system simply did not know what to do with a young child who had committed murder.