Re: Child Themes Not Automatically Being Assigned (by default) to New Blogs
What you’re describing is the expected behavior of BP/WPMU. BP is designed to work on a single blog in a WP installation. Additional blogs are more or less totally independent (“more or less” because BP still tracks activity that happens on those blogs).
You might be able to do what you’re trying to do with a plugin like https://wordpress.org/extend/plugins/wpmu-new-blog-defaults/, which lets site/super admins determine defaults on new blogs. You could set your home theme as the default.
If you want to offer child themes, you’ll have to build them. Make them children of your BuddyDress theme, make them available through Site Admin > Themes, and then your users will be able to select them.