Is it considered unjust for the eldest son to live in a separate room while his three younger brothers share one room?

Justice must be observed among children in gifts, not in expenditures (nafaqa). What applies to expenditures also applies to the issue of rooms. There is no objection to a father allocating a separate room for his daughter and grouping the sons in one room, or separating the eldest son into a room by himself away from his younger siblings.

It is obligatory to separate children in their beds from an early age, and it becomes incumbent when they reach ten years old, based on the Prophet's (peace be upon him) saying: "Command your children to pray when they are seven years old, and beat them for it when they are ten, and separate them in their beds." Also, it is not advisable for adults to sleep in the same room as young children.

Sometimes, the interest may lie in not having every child have an independent room, because Satan is closer to the single person, and it is better for more than one person to be together in a room.