//Para utilizar otro form seria de la misma forma

        private void adminToolStripMenuItem_Click(object sender, EventArgs e)
        {
            AltaAdministrador altaAdmin = new AltaAdministrador();

            altaAdmin.Show();
            this.Hide();
        }
        private void altasToolStripMenuItem_Click(object sender, EventArgs e)
        {
            AltaAdministrador altaAdmin = new AltaAdministrador();

            altaAdmin.MdiParent = this;
            altaAdmin.Show();
            //this.Hide();
        }