As far as my research have found there is currently no way to make a Parent and Child task with the 3den editor modules. Currently we have to rely on scripting to achieve this. The functionality is there, looking at you "BIS_fnc_createTask", so it ought to be supported by the modules as well.

I suggest either

1. Modifying the existing Create Task module to allow entry of a parent. A simple field in Attributes > System Specific would do. Syncing or grouping the modules could possibly be used to allow easy control.

or

2. Adding a new kind of Create Child Task module that must be synced or grouped to a Create Task module to work.