From 9 version of theme, all child themes were merged into core. You don’t need to install child themes separately now. Simply go to Theme option – general – Theme subset and enable subset which you want.
Please, note, that some of our demos don’t use any subset at all. Simply use Rehub and add colors which you need in theme options or in Customizer – Theme options
If you are looking for demo import, check our demo import guide If you want to import just one Homepage on site, check this article