You don't have any room to "stack" open curtains to the right of that right hand window. Are you just looking to have side panels to soften the room? Instead, how about Roman blinds in a fabric that works with your nursery theme instead/ That would leave a nice space between the windows for a couple of shelves or artwork.

I had a similar window in my house. I just ran the curtain rod right up to wall. I put 1 or 2 of the curtain rings on the wall side of the pole support, so the curtains sort of hide the support and stay over on the side. Nowadays I prefer the type of rod with no finial (or just a cap on the end of the rod). Just keep in mind that the curtain will cover a little bit of the window on the right. But honestly, I don't think anyone would ever notice.

There are also this type of curtain rod I've seen a lot lately:

On Ballard Designs, they call them French return. I think they work really well for blackout effect, which might be nice in a nursery.

https://hicks-house.com/2014/04/17/drapery-dreams/

What is the problem? You have plenty of room. As long as you can fir the finial on? You have no issue.

It's a nursery. I'd have done plantation shutters, and NO CURTAINS AT ALL : ) There is no need for curtains in a kids room, really

I was thinking the same thing - plantation shutters, clean and fresh. I put pretty pink sheer curtains in my daughter’s room and by the time she turned 2 she had pulled them down countless times. Lol Finally took them down for good.

My grandchildren use my custom made panels to hide in! Luckily the rods are study enough to hold them up while the are pulling on or hanging on them! I would do cordless cellular room darkening shades and maybe a pretty valence over it. Roman shades are nice too.

It appears you already have shades for privacy. Simple finish would be a valance. My nursery was very colorful so I would choose one that is fun and bright.