MBS will not bother to attempt to recover the money or take you to Court. It's not worth anyones while for such a footling sum.
Neither can you, nor they, prove that it was, or was not, your Grandson that downloaded their Software. As far as I can see, a number of actions have to be carried out to allow access to the site which includes clicking on Buttons "Affirming that you are over 18yo" and "That you have read the Terms & Conditions". I have little doubt that your grandson could have performed these actions but I think that MBS can have been said to have done about as much as is reasonable to transfer the onus of responsibility to the Viewer. To expect them to be able to put in place a system that someone who is unable to read could understand is, I think, asking more than is reasonably possible.