I have also tried to achieve this using the 'Child case types' section in the pyDeafult casetype rule. However I am also running into the issue where I'm not given the ability enter any dependencies/conditions to trigger the instantiation.
***Edited by Moderator Marissa to update platform capability tags***
There is a Pega-Case-Settings-Instantiation.pyShowCaseSetting When rule that controls when the Instantiation settings are displayed. You could check the criteria in this rule to see which condition is causing it to not be displayed in your instance.