.NET (C#): Getting child windows when you only have a process handle or PID?

public bool IsWindowActive(Int32 PID)
    {
        return IsWindowActive(Process.GetProcessById(PID));
    }

    [DllImport("user32.dll")]
    private static extern
        IntPtr GetForegroundWindow();

    public bool IsWindowActive(Process proc)
    {
        proc.Refresh();
        return proc.MainWindowHandle.Equals(GetForegroundWindow());
    }