any way you can post a current photo so we can get an idea of style?

are all those walls and doors staying?

if you make that pony wall a bit higher, he won't be able to climb it. as for removable, I'd say get all the furniture you are going to get and move it up in place prior to doing the railing. Doing a removable one isn't going to work here.

Also as soon as possible teach the kids to go down stairs backwards, it is the best defense against a tumble. My mom taught all of my crawlers how to do this.

How wide is the wall that the door to the bedroom is on? I have a slightly different solution but it may not suit you.

Put both of the babies' beds in the bedroom, along with all your clothes, bedroom furniture except your bed, kids' toys, etc.

Use this pass-through space as the area for your bed. It will be tight and it won't be terribly private but if you can add a door at the bottom of the stairs it will help, if privacy is a concern.

It sounds like these sleeping arrangements are relatively temporary.

If this idea doesn't work for you, the next best thing is a railing in place of the wall. I know lots of families who have raised lots of kids with these types of stairways in their main living areas. I don't know of one who has had a child go over a railing. It doesn't mean it couldn't happen, it would just be very rare. And like greenfish said, in any scenario teaching safe stair use is your very best defence. Your toddler can learn to do stairs now if s/he doesn't already know.

You know, since your son is good at stairs and you're concerned he might go over the gate you could put a gate about four steps down instead. My BIL built a climber-proof gate for his stairs by hinging a piece of plywood to the newel post and adding a gate latch near the bottom of it on the far side. An adult could reach and manipulate the gate latch. A child couldn't. It was too slippery to climb and too sturdy to push out. If your little one tumbled down four stairs it wouldn't hurt him, and the gate would stop him!

(I get that concern; I found my youngest 6' up a ladder that got left out--at age 13 months. He had and continues to have no fear.)

What about attaching track for sliding doors to BOTH the top of the half wall and the ceiling. Then you can install by-pass sliding doors cut to fit. They'd be easy to remove temporarily for moving furniture. They also would be easy to remove permanently. In the meantime you would have a solid barrier to keep the little from getting hurt.

Is there any space downstairs that you can convert for your toddler, since it doesn't seem as though this situation will last long? Just everyone live downstairs for now?

That is tough. Our last house had a half wall like this over the stairs (living room). My kids were never tempted to climb it, I don’t even know if they realized it was there. Totally differnt scenario leaving him alone in a bedroom with stairs like this, though. I’m not sure what I would do, to be honest. I think it might be difficult and/or expensive (and/or a lot of work) to find a truly safe solution which is also flexible (not permanent). If it is too flexible, a curious toddler would be able to figure it out, obviously defeating the purpose. I am not sure what your floor plan is like downstairs, but is it possible for everyone to be downstairs on the main? If not, I would put him in the bedroom on the main and get a good monitor if you are concerned about sleeping on a different level.

Here's another potential option . . .

Another thing to look at, safety netting:

https://childproofnets.com/

https://safetynet365.com/Staircase-Safety-Nets:::233.html

https://incord.com/specialty-division/stair-barrier-netting/

Remove the half wall and frame a wall floor to ceiling and then install carpet on the stairs. The wall and carpet can be removed at at later time. Maybe a new handrail.

The carpet will inhibit slipping on the stairs. If you install a gate at the top of the stairs, it should only swing over the floor, not the stairs.

I just did a similar fix for a nice young family in Portland Oregon.

@Classic Structures Inc. The OP had a concern with a permanent wall inhibiting getting furniture in and out.

If you cannot go with a full height wall then cargo netting wall to ceiling (very securely installed*) until the baby gate is removed.