Do the other wives have to be fully covered in hijab when this boy is present.
Jazakum Allahu Khair.
Praise be to Allaah.
The boy mentioned is young, and women do not need to cover their faces when he is present. Allaah says (interpretation of the meaning):
And say to the believing women to lower their gaze (from looking at illegal things) and protect their private parts (from illegal sexual acts, etc.), and not to show off their adornment except only that which is apparent (like palms or hands or one eye or both eyes for necessity to see the way, our outer dress like veil, gloves, head-cover, apron, etc.), and to draw their veils all over juyoobihinna (i.e., their bodies, faces, necks and bosoms, etc.), and not to reveal their adornment except to their husbands, their fathers, their husbands fathers, their sons, their husbands sons, their brothers or their brothers sons, or their sisters sons, or their (Muslim) women (i.e., their sisters in Islam), or the (female) slaves whom their right hands possess, or old male servants who lack vigour, or small children who have no sense of the shame of sex. And let them not stamp their feet so as to reveal what they hide of their adornment. And all of beg Allaah to forgive you all, O believers, that you may be successful. [al-Noor 24:31]
This little boy is a small child who has no sense of the shame of sex, so it is permissible to uncover one's adornment in front of him. Ibn Katheer (may Allaah have mercy on him) said in his Tafseer: The words (interpretation of the meaning), or small children who have no sense of the shame of sex refer to children who because of their young age do not understand anything about women or the temptation they may present, such as their soft voices or the way they walk. Because the child is so young, he does not understand anything about that, so there is nothing wrong with letting him enter upon women. But if a boy is approaching puberty and beginning to understand these things and to distinguish between who is beautiful and who is not, then he should not be allowed to enter upon women. And Allaah knows best.
Sheikh Muhammed Salih Al-Munajjid