As you already said yourself, the room definitely is too dark.
I don't think you have to go with a different colour palet if you don't want to. Who says all nurseries must be visions in pastel. But what you do need is balance.
In the current screenshots everything is the same tones of black and purple. If you go for such a colour combination it's important you add lighter tones to your palet to offset the darker ones. This will also make the black and the violet you do put in there pop much more and thus strenghten the mood you want to go for.
You can basically mix and match however you want, but my advise would be to lighten the floors, as well as the ceilings. Perhaps you can try a very light beige (yellow-beige) tone. Violet and yellow are opposites colours on the colour wheel and if you pick the right tone it will complement the violet extremely well. You could also go for an analogous colour palet, but blue/pink may not work as well for your room (because you're searching for very light tones), but you'll just have to try it out and see. Moreover, I wouldn't give all walls the same (colour) wallpaper. When you look at real life interior design you'll rarely find a room where all walls are purple, it's more common to create a focal point. The mood and look you're going for doesn't require everything to be uniform. Then as a last tip, if you go with a dark wall, I'd not go for dark furniture. In the current rooms everything's pretty much non-descript because it blands into each other. Rather, if you want your colour palet to pop you have to create contrast.
The balance between contrast and cohesion is what makes or breaks a design. It is the paradox a designer's never done solving.
I hope this was helpful to you.