I may be misusing the features in the Gantt chart window so please inform me if so.
When I have arranged a task and its subitems on a Gantt chart with their duration and dates but then want to move the task to a different date by dragging the task around, the subitems related to the task stay at the dates they were originally set to rather than moving with the task. There should be way to pin&unpin subitems to their task such that you can make them static or have them move with the task.