If you know the names of the child albums, then you can do what you are asking for. Imagine this scenario:

+ Parent album (set to private)
- child album 1
- child album 2

Create a role with edit permissions that apply to each of the child albums. Any users in this role will be able to edit the child albums but not the parent album. Also, they won't be able to turn off the private setting because the privacy setting is inherited from the parent, and they can't edit the parent.

I think the only scenario where this doesn't work is when you don't know the names of the child albums. For example, on the demo site I set up an album named Public Playground. I give users the ability to create and edit child albums but it would be nice to prevent them from editing the title/summary for the Public Playground album. Currently this is not possible.