John Adams said: "I wonder if it might be more interesting for your Grandmother to see the play first, without any pre-dispositions, then watch any of the films with her (because all of them are pretty good about including the books' characters and plot details) based on any questions she might have.
I think it might be interesting (for you, and her both) to begin at a point where the play stands on its own accordfirst,and then explore what are, in relation to ...Cursed Child,the "prequels"(?)"
This is actually what I did. I’ve never read the books, but I scored a cheap seat, and I’ll see almost anything if the price is right, so I went for it. I loved the play, and it inspired me to read the books. I’m on book 3 now. Intending to go see it again after I’m done reading to see if I missed anything!