Division?! Small room / steep slope / radiator

Are there really 60 cm behind the door? It doesn't look like it in the photo, but that can be deceiving. If yes, then I would do it as Ibdk14 has sketched, head towards the door, but then with a shelf between the bed and the door, so that you have a bit of privacy in the bed. If the child has such fears about the "inhabited" niche, I would consider adding lighting so that the low knee wall feels cozier. As it looks now, it is really dark and "huhuuuu." But a nice lighting, maybe an LED strip indirectly on the slope, which can be well realized with the shelf that is on the side with the low knee wall in Ibdk14’s plan. Then this niche will surely lose its scariness. Plan everything in the shelf with pull-outs – gives a lot of storage space and little to dust. The wardrobe then goes behind the door if the 60 cm is correct. Whether it has to be pulled out that far or whether the seating area at the window should be given a bit more depth can still be considered. Also, I would put the desk a bit more in front of the heater, not directly in front of the floor-to-ceiling window (later there will be just a cable mess, which is better hidden in front of a wall). Half of the desk still stands in front of the window under which the radiator begins, and in the bright niche next to the wardrobe and in front of the window you can wonderfully set up a cozy corner with a beanbag for reading and dreaming.

What I meant by the LED lighting - here is a rough sketch to illustrate:


It shouldn’t cost a fortune at the carpenter’s, provides great storage space and really nice lighting for the room.
If you want to illuminate the floor more, then attach the LED strip to the slope and also attach the cover from top to bottom, that is, to the slope, leaving some space between the shelf and the cover.

another shelf between the bed and the door,

I would take that open, so without a back panel. It still creates the desired privacy but doesn’t divide the small room as drastically as a closed shelf would.

Wow. I am totally overwhelmed and grateful! Thank you so, so much for your tips and advice!!!!
Between the wall and the door (frame) there are exactly 60 cm. That's why I would tend to go for a cabinet with a depth of 55 cm...?!
How deep would you make the drempelregal (32° roof pitch!)? Do you have something like that custom made by a carpenter?
And do you just stick the lighting on? On the shelf? Or the wall? Sorry... but my name says it all.... haha.
And would you really prefer to place the bed directly against the wall with the door... isn’t it better opposite? Then when you open the door you’re practically right in front of it?!?!

Oh great! You have answered everything in the meantime after all

Between the wall and the door (frame) it is exactly 60 cm

Also possibly consult the carpenter here as well, he can fit it exactly. I once saw a wardrobe solution where the back panel was omitted: a guide rail for a sliding door was attached to the ceiling and floor, and the side or inner walls of the wardrobe were fixed to the wall, which also saves a few centimeters and then it should fit.
But honestly, who plans something like that??? I would kick myself if I couldn’t put a standard wardrobe there because of a few damn centimeters. That’s really crap. Can you still catch the BU or architect? They should be whipped, something like this must not happen.