I have a requirement to launch a child case upon a user action (in an assignment) and show the launched child case in the same screen so that it is a seamless user experience. Any suggestions on how to accomplish this?
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.