The JQL function childIssuesOf() is effective to get the full hierarchy from an Initiative and down. However it is way too limited as it will only get the children of one single specified issuekey. 

A normal use case would be when you have several Initiatives that belongs together and should be shown in the same plan. An example would be to create a plan with all Initiatives having a custom field "Function" set to the same value. In this case the childIssuesOf function is useless since you would need to specify every Initiative issue in the filter. 

The solution would be to use a functionality similar to the script runner linkedIssuesOf function where you could specify a subquery inside the parenthesis. 

For example issuekey in childIssuesOf("Function = Audio") would be much, much more useful.