Access controls on a form from another form

Directly accessing the controls of another form is not object-oriented.

MDI is designed for windows of multiple documents, not multiple windows. I don't know the details of your requiremetns, but probably a better design would be to create class that is like a document and then the forms and the rest of the application can access the objects.

You can freely access contents of another form, but off course you should not manipulate contained controls directly. Rather, create a public method on the form, e.g. like this:

public void SetPanelVisible(bool visible)
{
        panel2.Visible = visible;
}

Now on the main form you must keep reference to the child form. Use that reference in the button Click event handler:
private void button_Click(object sender, EventArgs e)
{
        form2.SetPanelVisible(true);
}

Zoran