Gable roof floor plan, plan for the bay window

How so? As a cupboard and a dirty corner?

I think it's a shame that an expensive bay window, which could actually be a super nice room, remains unused...

But I have to write that for almost every bay window.

Yes, a corner cupboard is drawn in the parents' room. However, as mentioned, I can well imagine a dressing table (woman) or a bench or something similar here in front of the window.
In the child's room, however, you can really create a great workspace. Whether the OP does it this way or differently is of course open.
Floor-to-ceiling windows would naturally restrict any use again.

@OP: I would plan the living room so that the couch is placed against the inner wall - where the TV is drawn now - and put the TV on the west wall. That way you would have a) the option to look into the garden, b) create another exit to the terrace, and c) possibly also watch TV from the kitchen or dining room if desired. I would place the dining table crosswise.