[curt.clifton]

This isn't a function of how "GTD is supposed to work". The Getting Things Done methodology isn't that specific.

This is a consequence of the fact that children in OmniFocus are assumed to be the actions necessary to complete their parent. (You may have different assumptions about the relationship of children to their parent, but if so you'll find yourself working against the tool.) Based on OmniFocus's assumption, it makes no sense for the due date of a child to be later than the due date for the parent. OF partially enforces that assumption by marking the children as overdue if the parent is overdue. Stronger enforcement would disallow setting the due date on children later than the due date on their parents. There's been some talk about doing that, but it would make some editing operations more cumbersome.