Ibn Sirin (may Allah have mercy on him) says that Qadi Shurayh (may Allah have mercy on him) was asked about selling the children of a mukatabah, and he said: "The children are like her: if the mukatabah is freed, the children are also freed; if she is a slave, the children are also slaves."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21700