Does the use of IncludeOn (in regard to a parent page we'll call ParentPage) imply that all other child pages must be explicit about being included on ParentPage, too?

Probably quite a confusing question - to clarify:

Under ParentPage, I can create a page of type ChildPage, and AnotherChildPage.

Yet if, on ChildPage, I specify IncludeOn = typeof(ParentPage), I can no longer create AnotherChildPage under ParentPage.

It's as though the use of IncludeOn has enabled something with regard to ParentPage (i.e. well - one page has specified this as a parent they must be included on, so now all other child pages that would like to be included on this parent page must explicitly specify).

If not, then is this not a bug?