Division?! Small room / steep slope / radiator

What does it mean to push the bed in from the far right?! Into the room? So then centered?

Yes, that would probably be somewhat centered then
I would probably build a knee wall myself: simply with wooden slats or more elaborately, so that the space behind can be used for storage. Possibly just low shelves at 50 cm height and the bed in front of them.
Then maybe an indirect light strip, so that your child is not afraid to sleep under the slope.
Just draw the floor plan and section to see if it fits with the existing bed and space.
Or what about the front wall to the left of the door then?

Have a look at [Drempelschrank]. This way, you can use the slant as a cupboard and the rest remains for the bed and play.

Or insert the dormer afterwards. It’s not rocket science either. However, furnishing the space often doesn’t become much easier because of that. But depending on the size, the bed can certainly be placed there.

without a dimensioned floor plan, it won't work, sorry.

Building knee walls is a great idea. Does the location of the heater or the construction type bother you? Better-looking ones can be found, for example, at tubes radiatori from Italy.

Good that you understood the floor plan… I also measured it again… The room actually has an area of 5.40 × 3.45. The 5.40 measurement goes all the way to the corner/knee wall.