integrate a textbox in a childform of multiple document interface form

Question

        TextBox tb = new TextBox();
        public string Texto
        {
            get
            {
                return (tb.Text);
            }
            set
            {
                tb.Text = value;
            }
        }

        private void OpenFile(object sender, EventArgs e)
        {
            Size = new Size(400, 600);
            tb.Parent = this;
            tb.Dock = DockStyle.Fill;
            tb.Multiline = true;
            OpenFileDialog openFileDialog = new OpenFileDialog(); 
            openFileDialog.InitialDirectory =   Environment.GetFolderPath(Environment.SpecialFolder.Personal);
            openFileDialog.Filter = "Textdateien (*.txt)|*.txt|Alle Dateien (*.*)|*.*";
            if (openFileDialog.ShowDialog(this) == DialogResult.OK)
            {
                string FileName = openFileDialog.FileName;
                string filetext = File.ReadAllText(FileName);
                Form childForm = new Form();
                childForm.MdiParent = this;
                Texto = filetext;
                tb.Text = filetext; 
                childForm.Text = Texto;
                childForm.Show();
            }
        }

I tried to integrate a textbox, but I didn't succied. Can somebudy help me with this application.

Some code in mdiparent1.cs: