Use the copy to library function, available by right-clicking on a book. That copies everything known about the book to the other library. You do need to switch between 'your' and your 'grandaughter's' libraries once to get the library name into the destination list.
You can also do it using save to disk. Save books with metadata (opf included) one book per folder, then in the other library, use add books, one book per folder. In this instance, calibre knows that the opf file (the metadata) is associated with the book and will fetch the info.
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