child form

  System.Windows.Forms.Form CurrentForm; // defined in the class declaration

        private void menuItem1_Click(object sender, System.EventArgs e)
        {
            Form2 FormToLoad;
            FormToLoad = new Form2();

            LoadForm(FormToLoad);
        }

        private void menuItem2_Click(object sender, System.EventArgs e)
        {
            Form3 FormToLoad;
            FormToLoad = new Form3();

            LoadForm(FormToLoad);
        }

        private void LoadForm(Form TheForm)
        {
            Point pp;

            TheForm.MdiParent = this;

            TheForm.Dock = System.Windows.Forms.DockStyle.Fill;
            pp = this.splitter1.Location;
            pp.X += this.splitter1.Width;
            TheForm.Location = pp;

            TheForm.Parent = this.panel1;
            TheForm.Width = this.panel1.Width;
            TheForm.Height = this.panel1.Height;
            TheForm.MaximizeBox = false;
            TheForm.MinimizeBox = false;
           
            if (CurrentForm != null)
                CurrentForm.Close();

            CurrentForm = TheForm;
            TheForm.Show();
        }
I think the above code will help you.