I like that. It wouldn't even have to be the same shape and size, just a volume of 7/7 water that would fill the room below to 4/7 height. That would even be doable without levers so it's suitable for dozens of separate rooms.
That design has problems:
1) all bedrooms to be filled must be 7 squares in size. So it's not compact or targeted at the single bed level.
2) needs multiple levers per room.
you could link 1 lever to both input and output water channels, but if you want to reuse the same reservoir for multiple rooms it needs to be fillable while a room is still in use, hence you need the ability to seal the input valve whilst a room is still in use. So that's 2 levers per room. and you have to time flipping the "off" lever for the water so that the room doesn't overfill. So you also need to have a separate master input lever for the reservoir so you don't have to time it. But now you're up to THREE levers (2 per room (fill and empty) and 1 master water valve) for full control of each single fillable room.
So, with the shared reservoir idea the steps to fill "bedroom #1" would be:
1) close the mater reservoir floodgate lever. - seals input of reservoir
2) close bedroom #1 drain lever - which will stop the water from just flowing right out
3) open bedroom #1 floodgate lever - lets water flow into bedroom
4) close bedroom #1 floodgate lever - once water has settled to 4/7 level.
5) open the mater reservoir floodgate lever - to allow reservoir to refill
5 f***ing lever pulls to fill one bedroom and put the reservoir back into it's initial state. No thanks.
Here's my solution, which combines a reservoir with the bedroom in a modular fashion, and only needs 1 lever per room ever, and operation of each room is entirely independent:
#~# < water source
#X# < floodgate 1
#~#
#~# < reservoir area
#~#
#~#
#O# < 1x1 raising drawbridge
### < wall grate or fortification, minimize chance of kid getting squished by drawbridge
#B# < Bed
#X# < drainage floodgate
### < drain tunnel / grates
you hook 1 lever up to both floodgates and the drawbridge. "position 1" of lever opens both floodgates and raises drawbridge, simultaneously draining the bedroom and refilling the tank. "position 2" of the lever closes both floodgates and opens the drawbridge. This separates off 4 x 7/7 water which settles into 4/7 water in the 7 squares between the floodgates. Use floor grates in the drain area, and that can double as an escape tunnel for any kid silly enough to get stuck in there.
With the above set up, most of the area can be floored over so only the 1x1 bedroom is exposed with ramps to climb in and out, and it gives individual control of flooding and refilling each bed.