Imam al-Shafi'i (may Allah have mercy on him) mentioned two views about them (the children): "One of them is that they have her (their mother's) status: they become free when she becomes free, and remain slaves while she remains a slave."
He (Imam al-Shafi'i, may Allah have mercy on him) said: "And some of the people of knowledge have said this as well."
Abu al-Nadr says that Abd al-Rahman bin Ya'qub was a mawla (freed slave) of al-Huraqah, of the tribe of Juhaynah. He says: My grandmother's master married her to a slave of his, and afterwards made her mudabbar. She had children after her freedom (was so declared). Then her master died, and the dispute was brought to Sayyiduna Uthman (may Allah be pleased with him), and he ruled that the children born before she was made mudabbar are slaves, and the children born after the tadbir are free.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المدبر / 21583