Supposititious children are fraudulent offspring. These arose when an heir was required and so a suitable baby might be procured and passed off as genuine.

This was a common concern in the classical period

Classical antiquity

Classical antiquity is a broad term for a long period of cultural history centered on the Mediterranean Sea, comprising the interlocking civilizations of ancient Greece and ancient Rome, collectively known as the Greco-Roman world...

 as there were dealers in supposititious children who would provide them for a fee. Laws were passed to counter this and those found to be illegitimate might be sold into slavery

Slavery

Slavery is a system under which people are treated as property to be bought and sold, and are forced to work. Slaves can be held against their will from the time of their capture, purchase or birth, and deprived of the right to leave, to refuse to work, or to demand compensation...

.