Could you take some pics from further back so we can see the whole space and how it realates to the house. Just add the pics in a comment

These were the steps we removed

This is the back of the house where This is located

The deck and the stairs where the child is standing doesn't look level. You could use the timbers from the steps on the right of the picture to make a few terraces on the slope and eliminate the last few stairs. If they were 2 or 3 timbers high and filled with plantings, a veggie/herb garden etc.

Sorry probably not a great picture angle. The stairs with the child are newer (last 4-5 years) and are still level and in good shape ( do need a cleaning & restained).

I was trying to think of a way to kind of fence off that space or just make it “pretty” but not usable. Don’t want the kids walking up their due to the retaining wall and steeper slope.

I thought about a garden but we already have a large area in front of the house fenced and ready to be planted from when my in-laws lived there last year.

A block planting of any small shrub, one that doesn't get taller than the surrounding fences would fill the space just fine. To get suggestions for that you will have to give a zone and general location.

Install orange plastic "construction fencing" along top of retaining wall using rebar as posts. To you the slope is an eyesore, to kids it's the best thing ever. Yes, they will get filthy but, just hose them off (clothes on) before you let them in.