what type of guard rail is needed on an open part of a 5 tread stair
For new construction, over 3 rises requires a railing to code (can't pass a 4" sphere). At a minimum I'd think you'd want a continuous handrail (extending to a newel post) for safety. I had a similar situation in my previous home, and I did exactly that. It was never a concern for small children in my case, and the inspector at sale didn't flag it. 

If you can't switch your rail to that side, consider a newel post and rail to the end of the wall, even without balusters.