try this
//Form parent
public frmMenu()
{
InitializeComponent();
pnlHide = panel1;
}
public static Panel pnlHide= new Panel();
//Form Child
private void frmChiled_Resize(object sender, EventArgs e)
{
if (WindowState == FormWindowState.Maximized)
{
frmMenu.pnlHide.Hide();
}
else
{
frmMenu.pnlHide.Show();
}
}