How do I make sure only 1 child window opens?

There's possibly a better way to do this in c#, but the logic I'm thinking of here:

a. Create the instance of the form as global to the class.
b. On the button click, just call the object.Show().

If you wanted the functionality to close any existing form, and then re-open a fresh one, you could check if the object is currently displayed, then close it, then open it again?

Hence:

class MyClass
{
   MyForm myForm1 = new MyForm();

   void OnButtonClick()
   {
       // simple solution...
       myForm1.Show();
   }
}

Hope it helps.