hopefully I’m understanding correctly, if not, please explain.
Usually, with a child theme, it’s inheriting the functionality of the parent theme, so that means you should be able to use the parent theme’s page builder.
I would copy the extra lines of css you added into an external file for safekeeping, as, with some themes, they will sometimes be lost when you change to a child theme.
The child theme would just be used to code additional styles or change page styles without touching the actual code files in the parent theme.
If you’re only going to use the admin interface, and not code anything, you probably don’t need a child theme.
If you want to change the layout via changing the html/php/js involved in the templates, then you’ll want to add a child theme, copy the files you want to change into the child theme and edit them there, then the system will pull from them there.
Make a little more sense?