פירוש על קידושין 43:11

The Torah says that when the slave goes free, his children go free with him. This can refer only to children born to the Jew before he became a slave, for we already know from Exodus that those born to him in servitude (children of the Canaanite slave given to him) do not go free. But the problem is that the children were not sold. Only the father was. So why state that they go free? To solve this problem R. Shimon rules that the master is responsible for the upkeep of the children while the father is a slave. Once the slave goes free, the master is no longer responsible for them. This would be yet another way the rabbis write this law out of existence—why would anyone take on the responsibility of maintaining not only the slave but also his children? It would be far cheaper to hire a servant.

The same rule applies to his wife. Again, this can only refer to a wife he married before he was a servant. The Canaanite wife given to the Hebrew slave does not go free with him.