I would put in a piece of glass - it's pretty simple and can always be removed.

Yes to Debbi’s advice above. My son and daughter-in-law had the same type of opening over a staircase. They contacted a local shower door company that took care of the opening making it safe.

I didn’t even think of glass, thank you Debbi.

Another idea that still allows for light, perhaps less expensive than custom glass and doesn’t have to be cleaned all the time Endure slays are wider than shown for a more secure set up

Suggest also painting the trim the same color as the walls so opening feels less window like

Ask the glass company for safety glass. It can withstand some force but still be transparent.

Perhaps a laser cut metal panel or wood panel made to measure, as long as it is safe, without big gaps to have hands stuck etc. I know where you are coming from. I have worked over 20 years with children with disabilities and loved it, special loving people

You might consider plantation shutters that are fixed in frame so they can be closed. The levers can be open for light, but the shutters can’t be moved.

Thank you for the great ideas! I will look into all of them.

If this is only a temporary issue, I’d just install plexiglass. It’s not going to look the absolute best, but it would Be the cheapest solution.

Or even easier, baby gate/yard.

Since this is only temporary and maybe you don't want anything there when company arrives, consider these plastic spike strips they use for birds and other animals. it's just a deterrent.

In my opinion, those plastic spikes are not appropriate for this application at all!!!

I guess the poster has to decide if a deterrent should cost thousands or a few dollars

I agree with a glass divider. If you want light to come through, make it clear safety glass. If you want to be able to open it, make it a pair of sliders in a track. It must have a locking mechanism up high where your daughter can’t reach it. Don’t put any furniture below it, nor anything else she could climb on.

Beverly I think the OP said it was to keep an autistic toddler from climbing up. I always enjoy reading your advice. I must be cranky today.

rustynail, we're all cranky these days. No worries and I knew I would get some backlash with the suggestion but all children would be tempted to explore that open wall, and thankfully kids grow up and understand more about dangerous situations. I just thought the idea was worth exploring.

There is both a safety and aesthetic issue here. The idea of tempered glass seems the easiest and offers the most light. I agree that ANY toddler would try to climb that wall.

Thank you for all the ideas. I am leaning towards the glass. Forgive me if this is a silly question, but how would a glass company/contractor install it? Flush against one side or in the middle? Would there be a need to remove the trim? Thank you again, all the advice is appreciated.

We have a half wall around our stairwell, and our twins never tried to climb it, so it is not true that ALL toddlers will try. I also worried about it, until it became clear that they weren't climbers (ironically, as adults, they both love rock climbing).

I would go with safety glass.