this.FindControl("MyDialog").ControlAvailable += (s, e) =>
{
    var screenChildWindow = (e.Control as ScreenChildWindow);
    screenChildWindow.Opened += (s2, e2) =>
    {
        var content = screenChildWindow.GetChildrenByType<ModalWindowDialogContent>().FirstOrDefault();
        if (content != null)
        {
            // do something
        }
    };
};