We put this question to Shaykh ‘Abd-Allah ibn Jibreen, who replied:
They will no longer be counted as being alone together (khulwah) if there is a third person present with them, whether he is their child or someone else’s, and whether he is the child of zinaa or the child of a legitimate marriage – on the condition that he is above the age of discretion and is of sound mind, that there is no fear of fitnah, and that the woman is wearing full hijaab.