I want that room! Those skylights are great.

Am I correct in assuming that the low part of the slant is significantly less than 6 feet high?

Yes,a secret bookshelf door would be next level cool.

Re:beads.I think you just need to attach the track. Also,a curtain could hang from a short ceiling rod.

https://www.shopwildthings.com/beaded-curtains-buyers-guide.html

I think a cool looking hiding door would be great. maybe wrap in a tree trunk wall paper.

Solar film windows. Buy at Home Improvement Stores.

I might have missed it - why anything is needed between the areas?

A tension curtain rod, or a decorative curtain rod, and attach a cute shower curtain to it? or individual strands of smooth polished beads that wont have sharp edges in case they go flying into a childs face. One time it seems like Pinterest had an idea that the took various items and made chains out of them. If nothing else, just get some soft lace curtain or sheer panels and use them on a rod. What a fun project!

The custom secret doors is a neat idea.

If costs is a factor, then a curtain rod hung. On adult side a neutral shade to match decor.

Good reviews. Other length / color options:

RoomDividersNow Hanging Curtain Rod With Brackets, Brushed Silver, 36"-56" Wide · More Info

i do not think that a hidden door would be difficult or expensive. ikea shelf on wheels.

I'm curious about something. Is there an egress window in that space?

Julia, please don't think I'm over reacting or criticizing but I wouldn't let my kids play in a room that didn't have a proper egress window. If there was a fire upstairs while the kids were playing, there's no window in that space to escape from. If you want to make the space safe for them, consider installing a proper window with a fire ladder so they can get out in an emergency if needed. I would also worry that the space isn't properly sealed off from the garage below. Finished space above a garage requires that the garage have 5/8" fire rated Type X drywall on the walls and the ceiling of the garage that is taped and mudded. It's to prevent any possible garage fire from moving too quickly to the floor above and also to prevent any carbon monoxide fumes from leaking up into the room above. It's just a word of precaution. Please take it in the spirit in which it was intended.

Hi Julia-

When your kids get to be 13 you may not want them to be entertaining their friends in a space you can't easily see in to. Since it would seem strange to all of a sudden take down the bead curtain or whatever you have put up, why not just leave off the covering? Even if the room is in a bit of a disarray, it is not downstairs where your company will see it. Besides, that it is open will give you the impetus to have them put their things away every day at the end of the day just like you do!

I was thinking this also. A office, sewing room, crafts.

You could use that space for yourself too.

Or you could let your kids use it.

Just be able to see in there when they are there.

Just like you wouldn't want them entertaining them in their bedrooms behind a closed door either.

Oh those pre-teen and teen years!

Although for a young girl those bead curtains are cool.

Check these out-

https://www.shopwildthings.com/curtains-and-columns.html

You would just have a problem taking them down, with a lot of drama about "don't you trust me?" and there is no drama like teen drama.

But, if your kids are still little, you could do a lot of glitter and glam and princess and superhero stuff now and put up those curtains, and then by age 10 say, "I know you've outgrown all of this now, right?" and take it all out, including the curtains. If you think they would go for that.

You know how compliant your children are.

You might consider something that defines the different spaces and gives at least the feel/illusion of privacy without the feel of being in a confined space.

Have you considered saloon doors? They wouldn't need to be a tight fit -- so no need for them to be custom made. They wouldn't block air flow -- so wouldn't interfere with heat/ac. They'd enable you to hear what is going on while letting the tweens be on their own. They could be solid or louvered and cover as much or little space vertically as you choose. You could probably even make your own with a couple of wide shelving boards and the right hardware. You could add whatever design appeals to the tweens.

https://www.custommade.com/western-style-saloon-doors/by/SCWW/

https://www.gettyimages.com/detail/photo/saloon-doors-royalty-free-image/557174393

You'd not even need to address the odd top angle -- just ignore it.

I think it's a fun space & needs something like a butterfly chair in corner.

Leave as is and paint some fun colors. No need for doors (expensive). Curtains or beads might just get ripped out as they run/walk by and it gets stuck on their clothing or whatever piles they are carrying. LOL. Those beads are so cute but they always follow you 10 feet out. How about some fun paint, big throw pillows/beanbags for the floors, small couch, string lights and call it a day...so fun!