If child process exits then there is no impact on the parent and if parent exits then child process becomes 'orphaned' (i.e. its parent becomes init - PID 1).
In case of thread their is no parent child relationship for example if thread A creates thread B and then thread A terminates, then thread B will continue to execute. [The exception to this is when the main thread (that is, the thread that runs the main() function) terminates. When this happens, the process terminates and all other threads stop.]