I believe that underlying support for the ability of clients to belong to more than one company exists (Or at least, a client login working for multiple client accounts across companies), however, there is more work involved in handling this. So, it's something we anticipated, but haven't had many requests for.
I'm not sure I understand the parent/child company relationship that you describe, or whether that's something that's even feasible in the current architecture. I suppose I'd need some real-world examples of how such relationships between companies would play out.