public partial class RadForm1 : Telerik.WinControls.UI.RadForm
    {
        public RadForm1()
        {
            InitializeComponent();
            this.IsMdiContainer = true;
 
            RadForm form = new RadForm();
            form.Text = "MDI Child";
            form.MdiParent = this;
            form.MinimumSize = new Size(800,600);
            form.Show(); 
        }
    }