MDI and it's children

When my MDI app loads, it shows a child form centered. This form has buttons as choices. When a button is clicked, that form closes and another opens.

From the second window, if they click the X to close the window, I want to show the original form again. If they click a button on that second window, I do not want to show the original form.

I am lost how to accomplish this. How do I know when the X is clicked to close the window. Or can I disable it and provide my own close button?

Thanks in advance!

~ Lisa