It is possible you are trying to make a parent event smaller then total duration of the child events. You would need to reduce the duration of one or more of the child events first. Then reduce the time of the parent event. You can also adjust the parent and child events simultaneously by holding the Alt key down when clicking on the parent event. This will select both the parent and the child events. Now resize the parent by dragging the edge with the Resizing Tool.
Tag: Outline 4D, StoryView