I was that child.
I found it incredibly difficult to not be recognised, to be talked about in front of others without my parent realising I was present, to be treated as a stranger who was intruding, to never find any common ground for a conversation, to not be able to offer any comfort because I was a 'stranger'.
Ratter, you have had ample training and much experience in dealing with dementia patients. Thankfully many of us children of dementia sufferers are inexperienced in dealing with the condition and find it painful, exhausting, deeply saddening, frustrating, consequently we don't all know how to cope. Some of us do and some of us don't.
I didn't cope and I hope I was never judged by the staff for not being able to cope. I did what I could and took what little solace I could from knowing I had found and was paying for a good nursing home that had a staff that could do what I could not.