A Borg device in which assimilated infants and children are held while their growth is accelerated and implants are added, until they emerge as adult drones.
The Borg do not raise children. Young captives are sealed in maturation chambers, where their bodies are forced quickly towards adulthood while cybernetic systems develop. A nursery of infants already fitted with implants was seen aboard the first cube encountered, in the Next Generation episode Q Who.
Seven of Nine, assimilated as a small girl, passed through such a chamber and remembered nothing of growing up. In the Voyager episode Collective, the ship meets a cube whose adult drones have all died of a pathogen. The only survivors are five youngsters who left their chambers too early and are trying, badly, to behave as a Collective.
Four of them, Icheb, Mezoti and the twins Azan and Rebi, come aboard Voyager and are placed in Seven's care.