I think it should not be possible to make a child of "BlogPost" anyhow, because of $allowed_children = "none". But it is.
In the CMS->SiteContent I click "Create" and choose "Page" from the list of possible new pages. But I do not click "Go" button. Then I choose a page of my BlogPost class. Then I click "Go" button near "create", i.e. in the upper left corner. The new page is created as a child of the "BlogPost". Or at least it is shown as a child of "BlogPost" in the tree in CMS (even after browser reload).