You would only 'own' the child of a slave until the child is old enough to say they want to be free. Also what makes you so certain that so many people would desire to sell themselves into slavery? Perhaps a person in a third world extremely poor country would be willing to sign a contract saying that in return for food and shelter they will do whatever is asked of them by the master they sign themselves over to. But I would have no reason to sign such a contract. Additionally, in return for their slavery the example person would receive a higher quality of life. Why would someone voluntarily sell themselves into slavery unless they get a large benefit out of it? So really it is hardly even slavery. Slavery means that you force a person against their will to be your servant. There is no such thing as voluntary slavery, so it is a bit of a misnomer.