//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();
}