I have a parent case and multiple child cases which get instantiated from the parent case using create cases smart shape. I add the "Update a Case" smart shape in the parent case flow in order to update all the child cases with some property value from the parent case.

I select "All child cases(and descendants)" in the smart shape and provide a Data Transform to execute.
Initially the child cases did not get updated and I had to debug the issue. I had to make the following changes for it to work:

1. The Data Transform even though is prompted from the Parent class , during execution it expects the Data Transform from the Child class. So I had to create the Data Transform in the Child case.
2. In the Data Transform I am forced to define "WorkObject" page for child class and "pyWorkPage" as the parent class.
Only then the functionality works.

I have attached some sample screenshots.
IMHO this requires some product enhancement as the smart shape should be more flexible and easily implementable.
Having written all this - Please let me know if I am wrong in my implementation.