Error: Non-assignable model expression: undefined (directive: childOne)

As a follow-up still regarding this plunker:

plunker

If I review the console in Chrome when selecting an item in Child 1, I get the following error:

Error: Non-assignable model expression: undefined (directive: childOne)

I am completely baffled as the directive element is an element, not an attribute and designated as such in the directive itself.