Thank you for your response @ensan-hcl. If I understand your suggestion, \.isChild.! is what I did pitch, yes, although @toph42's suggestion of !\.isChild agrees with me.
While I do not understand your question (perhaps a complete code sample would help), !\.isChild was not in proposed in the previous thread from what I can tell.
However, after your comment @toph42 did write out an example using the syntax you wrote was proposed in the previous thread. Applying unary operators to KeyPath directly when Value can use the operator, to use his precise words, sounds like a great idea to me.