I would like my plaster board ceiling to be raised in my children's bedroom as we are installing a loft bed and do not have a comfortable amount of space between the bed and the ceiling. Is there any way I can raise the ceiling without having the joists visible? Am I able to raise even half of the ceiling 600mm (about 2 feet)?
If I am reading your post correctly you want to raise the ceiling by 600ml's which, if Google is right, that comes out to 23.622In. I don't see that happening without a total rework of the ceiling. You have a better chance of lowering the bed at the bottom, but you are going to lose that space. Take pictures or give us a drawing with dimensions of the room and lets see what the group can come up with.
The answer to this question depends on how the existing ceiling is constructed.
Ceilings are often made with dry wall material that is nailed or screwed directly to the bottom of the overhead joists or rafter stringers. In this case there is little to nothing that could be done raise the ceiling without a major reconstruction of the building structure.
Sometimes the ceiling material (often drywall) is screwed to stripper boards that are in turn fastened to the bottom of the joists. This is usually done when there is a wide rafter spacing that is wider than recommended for mounting for the ceiling material. To work around that issue these strippers are mounted at right angles to the rafter stringers but at a closer spacing. An example would be a 24' rafter stringer spacing and the right angle stripper board spaced on 16" centers. It may be possible to raise such ceiling a small amount (by the thickness of the stripper boards) by removing the strippers and replacing it with blocking between the rafter stringers and then mounting the ceiling material to the bottom of the stringers and added blocking.
There are some ceilings that are made with drop in panels that fit into metal rails. The rails are usually mounted below the bottom of the overhead joists or rafter stringers by 2 - 4 inches to allow for the tiles to be slid into place from above the rails. If you have that type ceiling it may be possible to remove it completely and then install a regular drywall ceiling that screws or nails directly into the overhead joints or stringers. If you do that however do note that the drop panel ceiling is often installed for the purpose of being able to access the space above. Moving tiles is a lot easier than removing and replacing drywall.