Requirement: We need to call multiple child Process Flows using multiple SPAWN activities in a process flow and these Child Process Flows need to be executed in a way that when one child process flow completes it execution then only the other Child PF shall proceed for execution.
Solution: You can achieve this requirement of executing Child Process Flows one after other by using "Synch" action just after the Spawn activity. This Synch activity will wait for the child activity to complete its execution and then only allow the parent PF to call the next child flow.
Spawn with "wait for child" vs Synch
Spawn activity with "wait for child" true allows the Parent PF to be in running state until the child process completes its execution whereas, Synch activity allows the parent process flow to wait for the completion of the child process first and then proceed to the next activity after this spawn call.