I am new to both. I have installed and created twentytwelve on my local server. I am about to use Buddypress and create its child theme, too. Question is, how these child themes are related to each other now, in terms of editing? What happens if I edit one of them? I have difficulties grasping this concept (I understand the child theme creation, though).
Your question is not very clear, but you should for the most part think of themes as being very much distinct. There is only one relationship between themes-- that between a parent theme and a child theme.
If you edit a parent theme then anything in the child that depends on the code you edited will be effected. The reverse is not true. Editing a child does not effect the parent. If you edit a child the edit has no effect on any other child themes of the same parent.
Some theme frameworks may muddy this distinction, however.