static class ProcessExtend
    {
        // [StructLayout(LayoutKind.Sequential)]
        private struct ProcessBasicInformation
        {
            public int ExitStatus;
            public int PebBaseAddress;
            public int AffinityMask;
            public int BasePriority;
            public uint UniqueProcessId;
            public uint InheritedFromUniqueProcessId;
        }

[DllImport("ntdll.dll")]
        static extern int NtQueryInformationProcess(
           IntPtr hProcess,
           int processInformationClass /* 0 */,
           ref ProcessBasicInformation processBasicInformation,
           uint processInformationLength,
           out uint returnLength
        );

public static void KillProcessTree(this Process parent)
        {
            var processes = Process.GetProcesses();
            foreach (var p in processes)
            {
                var pbi = new ProcessBasicInformation();
                try
                {
                    uint bytesWritten;
                    if (NtQueryInformationProcess(p.Handle, 0, ref pbi, (uint)Marshal.SizeOf(pbi), out bytesWritten) == 0) // == 0 is OK
                        if (pbi.InheritedFromUniqueProcessId == parent.Id)
                            using (var newParent = Process.GetProcessById((int)pbi.UniqueProcessId))
                                newParent.KillProcessTree();
                }
                catch { }
            }
            parent.Kill();
        }
    }

static void KillProcessAndChildren(int pid)
{
    ManagementObjectSearcher searcher = new ManagementObjectSearcher("Select * From Win32_Process Where ParentProcessID=" + pid);
    ManagementObjectCollection moc = searcher.Get();
    foreach (ManagementObject mo in moc)
    {
        KillProcessAndChildren(Convert.ToInt32(mo["ProcessID"]));
    }
    try
    {
        Process proc = Process.GetProcessById(pid);
        Console.WriteLine(pid);
        proc.Kill();
    }
    catch (ArgumentException)
    { 
        /* process already exited */
    }
}