That isn't going to work, ever: the clue is in the
new
keyword. To access the form that is being displayed, you need to get the currently displayed instance, not a new instance - the new one will have nothing to do with the old. Think of it like a car: if you put your mobile phone in the glove box of your car, then go out an buy a new car would you expect it to have your mobile in it's glove box?
But even then, don't.
It's a poor idea, because it "locks" the design of the two forms: you can't make changes without considering the effects on the outside world, and you can't reuse your child form anywhere else.
Instead, your child form should ask the parent to do the job for it.
Have a look here:
Transferring information between two forms, Part 2: Child to Parent[
^] - it explains how to do it.