Get All Active User Tasks including the Subprocess(invoked through Call Activity)

Hi,
We are using a Flowable Engine embedded in a Spring boot application . We have a process model which has several Call Activity . Is there a possibility to fetch all the active user tasks of both Parent and Child process .

We are using the below approach : -
taskService.createTaskQuery().processInstanceIdWithChildren(parentProcessId).list();

but it is giving us empty list .

Any chance we can achieve this ?

Thank you