I agree with sallymo that fabric treatment probably would be the best choice for you. Mounting blinds above the arch would not look good. Blinds are good for light control and insulation. But they don't look as nice as fabric treatment. If these windows are not in your bedroom, you don't have to worry too much about covering them up with blinds. Ceiling to floor curtains look formal and can also provide light control and insulation when you close them. I have arched windows in my living room and dinning room. I have a layer of sheer fabric inside my curtains and I leave the sheer closed all the time for privacy and keep the heavy weight curtains open. If you want to mount the blinds above the arch, you need to cover the blinds with some top treatment at least.

OK, here's what I came up with. Went to Home Cheapo and bought 2 2'x8'x4" white foam boards. Needed 2 to cover the last 20 or so inches. Didn't have any long sheets of paper so I took an old full desk calendar and taped them over the window to create a template (both too the window and too each other). Used a sharpie to trace both the outer edge of the trim as well as the inner edge of the trim. I peeled the template off the window, cut along the outer tracing to reveal the outer shape of the window. I overlayed that to the foamboard and taped it in place. I used a utility knife to make the initial pass, then peeled away the template. Took the board outside and made a full cut thru the board using a drywall saw. BTW...DO THIS OUTSIDE, THE STYROFOAM GOES EVERYWHERE!! I re-layed the template over the board and cut the inner line representing the inside of the window. I wanted that cut to be about 3" of the 4" depth. I then turned the whole thing on its edge and made an end cut with the utility knife to create a rabbetted edge. I knew I couldn't cut a perfect inner shape of the window so I figured an outer edge would allow me to wedge the board into the window. The outer edge would cover the trim, hide any imperfect cuts (thus not letting in any lights thru cracks) and giving me something to gently grab when I want to remove it. It looks good not great. The foam board is a b*tch to cut. It does go EVERYWHERE...think about what those styrofoam packing peanuts do when you open a box. The outer edge is also pretty fragile. But it does block 85% of the light (some does come thru the board itself), and most importantly it cost me $22.

I would do some blinds on the bottom and leave the arch since this is a play room make sure the blinds have no cords.

@Ely M Beautiful inside too! Gorgeous floors. Love those windows & would be a shame to hide any of the features of them on inside. I’m guessing windows are centered on the wall - looks off in picture but guessing that’s angle. I’ve never seen angled rods as @threers suggests. I also think any inside mount blinds would be custom & $$$$ due to shape & I don’t think outside mount fits these windows. I would go with drapes with the rod just above highest point on center window & with panels that split in middle & pull half to each side. I see the vent at lower right so you may need to use shorter length that ends just below sill. Even though the other window just like this one is evidently in another room, if you want to cover it at night, use same kind of drapes so the view is the same from street & the back side of drapes should be white. BTW, love the hanging light too!