Hello Jeff.
Marking the project as a daughter project is useful to organize in the registry, and to understand which projects depend on others, but the spelling, rendering or other progress is not copied or synchronized, since, although daughter, it is a project with its own folder... but, what you can do is to copy from the parent project to the daughter those files that contain the approved spelling and rendering information.
For this, with care or with technical help, you can copy the files SpellingStatus.xml for spellings, hyphenatedWords.txt for hyphenated or TermRenderings.xml for renderings, from the parent project folder to the daughter project folder (the project folders are in C:\My Paratext 9 Projects\[project folder]).
You must close and reopen Paratext to see the changes.
You can repeat this whenever you want to update the information. Note that if you modify anything in the daughter project, these changes will be lost if you replace them with the parent project files.
Saludos,
Pepe.