Its not mandatory when the Parent case is resolved the child cases should resolve. If we use any Ticket like AllCoverResolved or pxForceCaseClose with the CloseAllSubcases flag setting to true then all the child cases will be resolved when the parent case is resolved. Parent Child relation is for dependency purpose like when the child updates if the Parent needs to be updated then in that scenario again Parent case should be re-opened so in order to avoid that Parent will be kept open until all the child cases are resolved.
Is it possible to close a child case when closing a parent case with the OOTB activity pxForceCaseClose and set the pyStatusWork of the child case in a particular state (i.e. Resolved-Cancelled)? Can I achieve this requirement using the Status for sub cases parameter in the OOTB activity pxForceCaseClose? If so, the status will be set only for the child cases not already closed, am I right?
One more point I would like to add on this, in the case type itself you can have a last stage "Resolved" or something. There on clicking on the resolved stage you will have a checkbox option "Resolve all child cases"-> "Close all assignments". If you have not checked that checkbox, when the case reaches the resolved stage, it will only resolve the parent case with this config.