Slaves can be trusted advisors, teachers, and companions of their owners, but they cannot plead a case and their testimony is inadmissible in a court of law. There is no obvious physical difference between slaves and freedmen. It's not a matter of race or even nationality. And record-keeping seems to be extremely sloppy. Hence it is easy to kidnap children and sell them as slaves in other cities.
Prisoners of war become slaves.
People in debt can sell themselves into slavery to pay off the debt. Criminals may be enslaved as punishment. And the children of slaves are slaves. However one becomes a slave, once one is a slave, one is treated as property that can be bought and sold and that is totally at the mercy of the owner. Owners can do whatever they please with their slaves, including hiring them out as prostitutes.