Hello, 

Can this conversation helps: https://collaborate.pega.com/question/open-child-case-same-tab-parent-case

If you are currently using 8.x Version then you can leverage the feature of using the Ajax Dynamic Container to load/display the work objects in the section where you are interested.

One the action set of the button, can you call cancel after calling above activity which created the case and then call Obj-Open-By-Handle and pass the child case reference? e.g.: -CreateChileCase

-Cancel

-Obj-Open-By-Handle (Pass child case pzInsKey)

Instead of writing your own activity, why not use the OOTB tools available such as pxAddChildWork and OpenAssignment?

After the above executes, your OOTB pyStartCase (of the child i.e Pega-Childclass) has run and now you need to display the same in the same window.

Provided your child case has been defined in the Parent Case under Processes, has an assignment and is routed to the current operator, use OOTB Open Assignment newAssignPage of type 'Assign--' (defined in your Pages & Classes) to route the newly instantiated child work object to the current operator and it should replace and/or render the assignment on the same screen. .

The above assignment should open in the same window/screen.

You can also use a create case smart shape in the Flow rule to achieve the same.