A Hollow Child is a robot capable of passing as a human.
Physically, they consist of a robotic skeleton covered by a living layer of skin. Such technology had been possible for some time before 2080, but had been made illegal. A key feature of the Hollow Children is that they are programmed to believe that they are human.
Unless they are scanned for metal, or their skin layer is removed, they are nearly indistinguishable from humans. This makes their creation and existence illegal according to Clause 21 of the New Geneva Convention.
Later on, it is revealed that female Hollow Children can mate with human males, leading to the creation of Hybrids, which are completely organic.
They are first discovered when one breaks into Bergen headquarters and approaches Bergen with a gun in hand. He threatens Bergen even when surrounded by security robots. After a brief confrontation, he peels the skin off his face, revealing his true nature.
After this happens, and several other Hollow Children are identified, the Joint Chiefs and Bergen suspect Yohji Amada of creating these robots. As such, they contact the IRTA and authorize the activation of a Rust Crew task force to bring in Amada, setting the stage for Binary Domain's events.