Have fixed amount from child tasks automatically summed and rolled up to parent task

Why is this feature important to you - When fixed amount of a parent task is zero, it does not calculate planned revenue based on rolled up planned hours. 

How would you like the feature to work - The parent task should automatically sum all fixed amounts from child tasks. 

Current Behaviour - Fixed amount remains at zero until the user opens the tasks and manually enters the sum of all child fixed amounts.