C#

private delegate bool EnumChildProc(IntPtr hwnd, IntPtr lParam);

[DllImport("User32.dll")]
private static extern bool EnumChildWindows(IntPtr hWndParent, EnumChildProc lpEnumFunc, IntPtr lParam);

private IEnumerable<Form> GetChildrenSortedByZOrder()
{
    List<IntPtr> handles = new List<IntPtr>();
    if (IsHandleCreated)
    {
        EnumChildWindows(Handle,
            (hWnd, lparam) =>
            {
                handles.Add(hWnd);
                return true;
            }, IntPtr.Zero);
    }
    List<Form> children = new List<Form>(handles.Count);
    foreach (IntPtr handle in handles)
    {
        Form form = FromHandle(handle) as Form;
        if (form != null)
            children.Add(form);
    }
    return children;
}