User permissions on child accounts work the same as parent accounts, with one exception. Participants with the Strategic Lead designation will be able to act as administrators over child accounts all the way down the tree.
In the example above, imagine that Nancy is a Participant with the Strategic Lead designation on Agency. She will gain administrative access over Clients A, B, and C as well as Brands 1 and 2.
Note that Nancy isn't a part of child account teams unless explicitly added as such. This means that Nancy will be able to perform administrative tasks like configuration and team building for client accounts, but won't appear as a possible task assignee or reviewer unless she's added explicitly to the team of an account.