There’s a long-running feature/bug in WordPress that prevents you from using unpublished pages as parents: that is, you can’t add child pages to anything that’s set to private, password-protected, scheduled, pending, or draft.
Inclusive Parents is a lightweight plugin to remedy that. It adds private, password, future, pending, and draft pages to the Edit screens’ parent dropdown in Page Attributes as well as the Quick Edit and Bulk Edit parent dropdowns. It also adds private and password pages to the Menu screen. Unpublished pages have their (status) appended in all cases.