private void toolStripButton1_Click(object sender, EventArgs e)
        {
            FrmIngresoMascotas m = new FrmIngresoMascotas();

            m.MdiParent = this;
            m.Show();
        }
        private void ingresoToolStripMenuItem_Click(object sender, EventArgs e)
        {
            FrmIngresoMascotas m = new FrmIngresoMascotas();

            m.MdiParent = this;//el formulario padre es "este"
            m.Show();
        }