Hi Patrick,
https://www.lawlessfrench.com/subjunctivisor/le-temps-que/
It is required, but why? There are a number of situations that require the subjunctive -- a couple for example -- Wish, Emotion, Doubt, Desire ...... In the text given in your query, seems to me that there is an element of doubt or emotion being expressed given that the brother needs to be watched while the beds are being made up. There is perhaps a concern that the brother may misbehave or come to some harm if not supervised.
Hope this helps.
Jim