Can not make a custom field in issue for my requirement!!!

How to make a custom field attribute value of redmine father task(issue) according to the custom field attribute value of son task(issue) ?

summarize： I want to make a new properties(custom field) in issue .As with “estimated_hours” in the role of the same properties, can make he custom field attribute value of father task(issue) according to the automatic acquisition . Have you had this problem?