Hi Shiju John,
you can not get the sub process instance id from the parent process. This relationship between process instance and sub process instance is not stored in a table.
But you can check the class SubProcessNodeInstance. This class represents a sub process call activity and start the sub process instance. See the following code:
ProcessInstance processInstance = ( ProcessInstance ) ((ProcessInstance) getProcessInstance()).getKnowledgeRuntime().startProcess(processId, parameters);
this.processInstanceId = processInstance.getId();
((ProcessInstanceImpl) processInstance).setMetaData("ParentProcessInstanceId", processInstance.getId()); // note: wrong process instance id
You can try to read the property processInstanceId from class or build your own logic.