C# – Maximize MDI child form

I'm working on a legacy WinForms MDI application and have some trouble making the child forms behave as I want.
My objective is to have the child form always maximized (docked).

The problem is, that even if I set MaximizeBox to false the maximize/resize button appears in the MDIs toolstrip and let the user resize (undock) the child form.
The only way to avoid this is to set ControlBox to false but then the close button disappears to (thats not what I want).

I've already tried to use a fixed FormBorderStyle and to maximize the child form when the resize event is fired but none of my approaches worked.

Is there any super secret property I have missed or is it just impossible?

Best Regards & Thanks in advance

Update

I wrote a sleazy method (thanks to @rfresia) for handling my child form, it may help others who run into the same issue:

//All child forms derive from ChildForm
//Parent MDI Form implementation
//...
private void ShowForm(ChildForm form)
{
    //Check if an instance of the form already exists
    if (Forms.Any(x => x.GetType() == form.GetType()))
    {
        var f = Forms.First(x => x.GetType() == form.GetType());
        f.Focus();
        f.WindowState = FormWindowState.Maximized;
    }
    else
    {
        //Set the necessary properties (any other properties are set to default values)
        form.MdiParent = this;
        form.MaximizeBox = false;
        form.MinimizeBox = false;
        form.WindowState = FormWindowState.Maximized;
        Forms.Add(form);
        form.Forms = Forms;
        form.Show();
        form.Focus();
        //Lets make it nasty (some forms aren't rendered properly otherwise)
        form.WindowState = FormWindowState.Normal;
        form.WindowState = FormWindowState.Maximized;
    }
}
//...

//ChildForm implementation
//...
public List<Form> Forms { get; set; }
protected override void OnClosing(System.ComponentModel.CancelEventArgs e)
{
    Forms.RemoveAll(x => x.GetType() == GetType());
}

protected override void OnResize(EventArgs e)
{
    WindowState = FormWindowState.Maximized;
}