You can’t really create a Child Theme for bbPress only.
But if you like to work with the Child Theme idea – I personally think it fails big time but some people really like it then just create a Child Theme if you would do the WP way.
http://codex.wordpress.org/Child_Themes
After that you simply add a folder called bbpress inside your Theme or Child Theme folder and copy-/paste the files over from your Plugins directory to your Theme or Child Theme directory.
wp-content / plugins / bbpress / templates / defaults / extras / All those files are general Theme template files and will be read first if you copy them inside your Child Theme folder
wp-content / plugins / bbpress / templates / defaults / bbpress / All those files are bbPress (forum) templates files and will be read first if you copy them inside your Child Theme/bbpress folder.
An image will show you better..
http://i.imgur.com/23E41RZ.png
and