What are the dimensions of the window in the room? Code typically requires that a bedroom have at least one window that is: 1) at least 5.7 square feet, 20 inches wide, and 24 inches high; 2) able to open at least halfway; 3) is no higher than 44 inches from the floor; and 4) must be accessible in an emergency. Check your local codes.

If the window is up to code. get a custom door to fit the opening. I'd put it at the front end of the "hallway".

Maybe a sliding barn door ?

What a bifold louvered door like what would go on a closet door?

There are two windows in that room and both meet egress requirements to be considered a bedroom and passed inspection by our city so all good on that front!

Barn door makes no sense -- the frame of the door stands substantially proud of the adjacent walls, and you don't want to leave it open across the TV. Or to have it rumble when you go in to check on a baby!

I'd just use a slab door and have it cut down to the size you need. Shouldn't be a crisis.

I agree a solid door cut to size and that window I see for sure does not meet egress anywhere . Is this space off the master bedroom ?

If you don’t want to pay for a custom door, you could do an inexpensive accordion door

accordian would be terrible for sound control!

Sound control? there’s a giant tv mounted on the shared wall with the nursery-

That opening is only 6 feet tall? I'd hang a curtain on a tension rod & the kid will learn to sleep through TV noise.

I’d post pictures from the inside of the room. A true door may still be possible but set back into the actual room. But hard to say without more photos