Since this will be temporary (you may find out that your kiddo as they get older doesn't like completely dark rooms - mine wants nightlights on even for naps), I would just recommend putting film on those windows. Also, get another curtain panel or two so the curtains look fuller when closed. That will help with the light seeping in on the sides. You could also raise your curtain rod; while the pooling on the floor curtains look cool, your kid will probably step on them and rip the whole thing down when they're older (accidentally, of course).

Thank you - yes we still need to get the drapes hemmed but haven‘t been able to yet due to COVID. We can’t raise the rod because of the angling of the ceiling on the left.

Our daughter is a newborn and naps much better in darkness so we would like as dark as possible for now - probably at least a year or two so willing to invest in a window treatment!

You could add a board mounted (headrail) jabot/valence above the drapes.

https://www.youtube.com/watch?v=QXz-BXKHrWg

Window film can be a permanent treatment. We use it in our bathroom windows. But it has the added advantage of being inexpensive, not clunky to use (like I imagine a tiny round window treatment would be). You could also get patterns to match decor when that changes (as it inevitably will). Stained glass ones are nice, too.

Are the current blinds long enough to be hung above the arch? That would help. Also, consider replacing the rod with a French return one to help with the light on the sides. Or, if you haven't already, perhaps you can simply hang the last curtain ring on the other side of the current rod bracket. You want the curtain to curve perpendicularly into the wall as much as possible. Does this make sense? I understand the struggle, btw. Had to get a blackout solution for my family room and it's not easy finding affordable stuff in stores.

We had same challenge with our twins. I ended up getting a rod that had a very low profile (didn't stick out far from wall.) That eliminated the halo effect somewhat.

That’s a good idea! May need to give up my preference for a pretty rod and just get something functional!

Can you apply a blackout film to the arch and raise and lengthen the curtain rods?

Arched rod so the curtain returns all the way to the wall edge. I have a similar window.