Marking a page as the child of another page is easy in OurBigBook Web: you can simply set the parent of the page directly on the editor UI.
If you don't want the article to be the first child of a parent, you can also set the "previous sibling" field. This specifies after which article the new article will be inserted.
Video 11. OurBigBook Web parent selection on the web UI. Source.
Video 12. OurBigBook Web error reporting. Source. Note that Video 11. "OurBigBook Web parent selection on the web UI" is not present on this video as the feature wasn't present when the video was made, but the error reporting remains valid.