Adds the given page as a subpage of the current page. This tag only represents the relationship, so body content is not allowed. It is permissible to have a child in a missing book.
To build the relationship between pages and subpages, the page should include <core:child> tags for each child page within the body of the <core:page> tag.
Due to optimizations during page-only capturing, the child pages must be added directly in the page body, not from within other nested elements.
allowChildMismatch is enabled on the <core:page>,
the child must also consider this page as a parent.