Reports within Reports

Two thoughts.
1. Create the report on the child table.  Lookup the Task Name down to the Sub Task and include it on the report as a column or a report "Group By".

2. Create a new formula Text field on the Sub Tasks table called Child table called [Sub Table fields for roll up].  Use a formula to combine the field you want to show on the Parent tasks table.

The Use a Combined Text Summary field to Roll that field up to Tasks.

Then use this formula to make them into an attractive vertical list for use on Task Reports. 

var text CombinedText = ToText([My Text Concatenation Field]);

// substitute new line for semicolon
SearchAndReplace($CombinedText," ; ","\n")