Change property on MDI parrent from child?

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C#

Hello there,

I have three forms:
frmMain, frmLogin and frmPanel.


When I run my program it shows frmMain which is an MDI container, and on load I open Login form as child.

Now when I login and information is correct I need to enable buttons on my toolstrip1 which are disabled on load.

What I've tried is to make a function inside frmMain

void functionOnForm1()
{
    this.toolStripButton7.Enabled = true;
}

then on login form after password is OK I call this function:

((frmMain)this.MdiParent).functionOnForm1();

but I get the following error:
'Project1.frmMain.functionOnForm1()' is inaccessible due to its protection level (CS0122).

This is frmMain:

public partial class frmMain  : Form
{
    public frmMain()
    {
        InitializeComponent();
    }

    void functionOnForm1()
    {
        MessageBox.Show("ddd");
    }
}

Does anybody know how to change property or labels from child on parent?