The solution is straightforward.
First remove all the children from under the parent and put them somewhere else in the hierarchy temporarily.
Then move your parent's position to where you want it to be with respect to the children.
After that, move all children back inside the parent with the help of Hierarchy view.
Since the editor won`t move your objects in the Scene view if you make some changes in the Hierarchy view, the children will stay in the same net coordinates. So, after doing that you will get the origin point of the parent in any place that's convenient for you.
