for a Dallas Cowboy Player in his/her window. Its a bench and a seat with piano hinges allowing the seat to hinge up and under that is toys and they picture albums and regardless what you put in it NOW it will hold something else later on. Its great for out of the way storage. I made it out of Oak Plywood, stained it, sealed it, and then she laid glen plad seat cushions over that and its great.
Now, with your register below, you will have some modifications and you do not want to restrict or choke the outflow of treated air from that register. You could add an extension to that register and bring the register to the outside of the box and mitre it in. You can just extend the floor molding and show molding out from its existing spot. No problem if done right.
Now something to consider, when kids sit on the bench will they accidently stand up and fall through the window. This is one big safety consideration. Kids will NEVER sit down as told.
Consider resale value also, will the next family like this?????