Nodehierarchy and access control... This webcast has a walkthru of NH; it includes the NH.Access module. I assume that he is using 6.x, as a lot of stuff is still very much beta'ish in 7.x, including NH.

I'm wondering if there is a module that gives me the ability to assing a permission to a node and its children.

My current setup is replacing a "Per-node" permission system, so the current Node Access grant is acceptable. I would like to progress into a "Per-Hierarchy" permission, so that a user can create children and maintain them without my intervention.

I just want to be sure I'm not missing a "related module" that's functional.
You might consider the Workbench Access module: It is a hierarchy based approach to grants; it might be a much better match for your use case, and it can use a number of different hierarchy management tools, such as the Menu system, Taxonomy, etc.