org-enforce-todo-dependencies

org-enforce-todo-dependencies is a variable defined in `org.el'.
Its value is

nil

Documentation:
Non-nil means undone TODO entries will block switching the parent to DONE.
Also, if a parent has an :ORDERED: property, switching an entry to DONE will
be blocked if any prior sibling is not yet done.
Finally, if the parent is blocked because of ordered siblings of its own,
the child will also be blocked.

You can customize this variable.