public partial class ChildForm : Telerik.WinControls.UI.RadForm
    {
        public ChildForm()
        {
            InitializeComponent();
        }

        private void radButton1_Click(object sender, EventArgs e)
        {
            RadMessageBox.Instance.LocationChanged -= Instance_LocationChanged;
            RadMessageBox.Instance.LocationChanged += Instance_LocationChanged;

            RadMessageBox.Instance.FormClosing -= Instance_FormClosing;
            RadMessageBox.Instance.FormClosing += Instance_FormClosing;
            RadMessageBox.Show(this, "Child Form message", "Information", System.Windows.Forms.MessageBoxButtons.OK,
                new System.Drawing.Bitmap(Properties.Resources.info), System.Windows.Forms.MessageBoxDefaultButton.Button1);
        }

        private void Instance_FormClosing(object sender, FormClosingEventArgs e)
        {
            RadMessageBox.Instance.LocationChanged -= Instance_LocationChanged;
        }
         
        private void Instance_LocationChanged(object sender, EventArgs e)
        {
            RadMessageBox.Instance.LocationChanged -= Instance_LocationChanged;

            var box = sender as RadMessageBoxForm;
            Point pt = new Point(this.MdiParent.Location.X + this.Location.X, this.MdiParent.Location.Y + this.Location.Y);
            box.Location = new Point((pt.X + this.Width / 2) - (box.Width / 2), (pt.Y + this.Height / 2));
            RadMessageBox.Instance.LocationChanged += Instance_LocationChanged; 
        }
    }