I've done the same. I bought a 高架床 and put in on top of the original 單人床, so that the two beds look like a 輥架床. The maid sleeps on the top (高架床) and my daughter on the bottom (單人床).
About 床褥床舖, the maid continues to use the old ones and my daughter gets new ones.
We also bought a bed railing to prevent our girl from falling off the bed.
Let her sleep on the floor with the mat that she used now. I do the same to my maid as it is better than 2 deck bed as the maid sleeps on the upper deck. They will sleep over and cannot take care of your kid during night as needed such as putting your kid to toilet etc.
In the morning, put the mat either under the bed or put it behind the door of the bedroom with hook and strings to prevent it form falling
We bought the 高架床 from Ikea. $990. Including delivery, installation, new beddings for my daughter, almost $2000. This is actually more expensive than buying a bunk bed from Ikea (last time the promotion price was $660 for a small bunk bed).
We cannot install a 子母床 because there isn't enough space to pull out the maid's bed. We can probably pull out only 3/4 of the bed because we have many boxes of toys in the room.
We also have the same problem, 1 side is window and the other side is aircon.
So aircon is right next to the 床腳, just 1 foot away from the maid's legs. We adjust the direction of the wind so that the maid will not be cold and some cold air will go down to the lower deck.
The 床頭 is next to 窗台. So our daughter gets window view and the maid gets nothing (there are no windows near to the top).
I used to have two maids. one maid takes care of my elder daughter and sleeps with her on the folding bed and the other takes care of my baby son. When my daughter grew older, I terminate the contract of my other maid and the one used to take care of my daughter sleeps in my son's room on the floor and take care of my son. There seemed no problems and it was 3 years ago.
So I don't think there should be any problem and it is not so unreasonable by asking your maid to sleep on the floor given HK's situation. Also, some of them in Philippines also have no bed and sleep on the floor.