@alex3715 D-class personnel are typically criminals on death row. (Meaning they've been sentenced to death.) Sometimes they also use other people, especially if the current situation demands a large influx of D-class personnel, but most of the time they're criminals who did very bad stuff.
In this case, they look for a criminal who commited violent acts of child abuse. Then they only need to combine that D-class with a regular human child of the appropriate age and BAM, they have an abused child that can serve as the host.
Really fucked up stuff.