Anything should work there with regards to light mitigation. You have a deep wall jamb so even a well measured and fit cellular shade would cut down on light pretty significantly.

Either a cellular shade, Roman shade, natural fiber shade or wood blinds would work to cut down light. As Windows on Washington said, you have enough room to do an inside mount on any of them. Curtains would need more room to stack back on the left side.

Is that a radiator below the window?

For a kids room I would do cellular shades with room darkening fabric. Inside mount and cordless.

Yes, it is a radiator below the window. It is a 100+ year old house.