The seat is like a carseat, but isn't very big. I used in back in February with my 9 month old and she pretty much filled it, so I think a 20 month old would struggle in it.
In club suite the carry cot/seat is attached to a shelf at the front left or right of the cabin. In club world (old) its attached to a drop down table at the rear centre pair. They are designated bassinet positions in the seat map.
You can't use the seat for taxi/take off/landing, they need to be strapped to you. They're only really useful for them to sleep during the cruise.
I wouldn’t think a 20m old would fit comfortably in it. I stopped using it when my son was a little over a year old. If you don’t mind using your points I’d suggest getting them their own seat.