I\'m running .exe file using this code:
Process proc = Process.Start(\"c:\\program.exe\");
proc.WaitForExit();
If I start Stopwatch>
Take a look at this - Find all child processes of my own .NET process / find out if a given process is a child of my own? or http://social.msdn.microsoft.com/Forums/vstudio/en-US/d60f0793-cc92-48fb-b867-dd113dabcd5c/how-to-find-the-child-processes-associated-with-a-pid. They provide ways to find child processes by a parent PID (which you have).
You can write monitor the process you create and also get its children. You could then track everything, and wait for them all to finish. I say "try" because I'm not sure you could catch very rapid changes (a process starting others and then dying before you get his children).