try block out roller blinds or venetians behind double curtains with liners hung separately to sheers but have them wide to allow the curtains to be drawn right back from the windows when you want the light and keep them just below the architrave away from the furniture and little fingers and consider rearranging the furniture so that during nap time the bed is facing away from the window on the darker side of the room and the cupboards are on the sunny side

Unless you are willing to replace the cassette blind with one with a block out fabric whilst still wanting to use shears in front as well, then I could suggest window tinting the glass with a suitable darkish film which can be removed at a later date when the child grows up and will not effect the use of the other blind / shears .
The front to back recess depth of the window frame will determine whether two blinds could be suitable fitted such as an added see through rollup sun screen blind and still be able to use shears as well.
The sunscreen blinds available have an approx darkness compared to the ultra dark window films (( approx 95% light reduction) and has worked fine in applications that I've dealt with .
Either that, too fit a sunscreen rollup blind that will allow light levels to be adjusted on the out side of the architraves ( inside the room) and have fitted fully across the window frame that extends either side of window frames by a few inches and have the fabric under rolled so fabric rolls closer towards architraves to reduce light from filtering in from the sides .
As I deal with both tinting and blinds I've often been able to solve customers problems ; provided they are willing to be flexible in there choices of products