At the moment there is not a function within Zoom for a host to monitor all their breakout rooms at the same time. It is a current limitation of the design. You have to individually go in to each room to monitor them. I believe this would be a great feature request though, and submitting a request at zoom.us/feed can place this request on the roadmap. Hope this helps!
I would strongly suggest adding an ability for the host to monitor breakout rooms simultaneously on one screen. Class has this ability, but it would be ideal if we could do this on Zoom! I hear the same request from many educators. It is time-consuming to visit each room, and you never know if everybody is working actively in the rooms while the host is not supervising.
I would like to support the request to monitor multiple breakout rooms at the same time. We have seen this feature with another product Class.com and have been hoping to see this as a base feature in Zoom. With our dependence on Zoom to teach our courses this would be a positive enhancement.