It happens rarely when the parent-child relationship is lost. Do you have any functionality where at a point in the flow of the parent case, the process is moving forward the child case and vice-versa?
add a resolution stage to set case status to Resolved-Withdrawn, then, go to this stage to withdraw case iso calling activity
in child case type, call pxForceCaseClose on page pyWorkCover (defined as parent case type class) to withdraw parent case, then, child case itself will be auto withdrawn too