I think a built in reading nook is a great idea. Your local home improvement store will cut your lumber for free. Mount 2x4’s along bottom of windowsill and the back and left wall in the studs using your drill and large wood screws. Allow for a 3/4” plywood top. Extend the top out 4’. The plywood should be paint grade and paint it out to match the sill. The back wall ledger should be in two pieces with a gap to allow you to purchase a plug in wall sconce on the back wall and a computer charger etc before assembly. Then the bookshelf serves as the end support for the bench. Have a cushion made in the size of the plywood top (24x48). You may run into a problem finding the correct height bookshelf for the end piece but wood shims at the back of the bookcase won’t be seen. Screw it all together with your drill. Leave the end support (bookshelf) unattached for access. Look for a couple larger pillows for the back wall and viola it’s greatness and your kiddo will love it