A few weeks ago I took my eight-year-old, Elliott, to see Harry Potter and the Cursed Child. It was the second play of mine Elliott has seen, after my adaptation of A Christmas Carol. Now, before I’m accused of being an egotist who only takes his kid to see his own stuff, I do want to say we’ve been to shows by other writers, many of which he’s preferred. It’s just that – for two different reasons – I wanted him to see both of these.

We went to A Christmas Carol when he was five. Spoiler alert for those who haven’t seen it, but there is a point when it snows inside the theatre and I thought he’d find this wondrous. What I hadn’t quite taken into account is that before that there is quite a lot of talking going on. It’s funny and moving – please come and see it – but quite a lot for a five-year-old to take in. He pulled on my arm, a few minutes before the interval, asking: “When will this be over?”

So taking him to see Harry Potter and the Cursed Child was nerve-racking.

Bedtime is my gig in our house, and I had read him all seven books. We cried together over the death of several characters. We laughed, we squirmed, we’d been terrified (I still maintain the end of Harry Potter and the Half-Blood Prince is a gateway drug for body horror.) He loves them more than anything else in his life, and still spends a good proportion of his time swooshing about dressed in robes.

The play, however, was something different. It is in evidence throughout our house. Christine Jones and Brett J Banakis designed the company’s wands and a picture of mine is in our bathroom. The producers Sonia Friedman and Colin Callender sent Elliott a Cursed Child babygrow which I have framed on my office wall. And the play, or my bit of it, is dedicated to him. The poor kid has been haunted by the thing. In fact, when I first suggested reading Harry Potter and the Philosopher’s Stone, Elliott refused. It took his cousin Buzz almost a year to persuade him that he might like it.

Elliott was one month old when we went into previews. The idea of him was alive through every stage of the writing process. It took me two years to write the play; it took us five to conceive him (and seven rounds of IVF). The play is about many things but a lot of it is about a father struggling to connect with his son. Harry Potter haunts Elliott and Elliott haunts Harry Potter.

We went with Buzz to see it. We chose a Saturday for the full-day experience (the best way to see it in my opinion). We arranged to meet the actors in the break between parts one and two. As the day drew on I went from rabid insecurity to such pride in the brilliant company of people we have doing the show and the way they give everything to make it special. It is hard to inherit a role several years after a production has opened but, honestly, they all dazzled. It felt like I saw it for the first time all over again. And Elliott loved it.

Actors are brilliant, as every writer will tell you. Or most will. Or the nice writers will. (A good rule in this creative business of ours is to never trust an actor who slags off the writers, and never trust a writer who slags off the actors – they’ll generally be cowards.) Actors who take over roles and find a way to own them are better still. Years ago I saw Marisha Wallace do so in Dreamgirls, taking an audience somewhere extraordinary. More recently, Elliott and I saw Cory English in Back to the Future: The Musical (told you I took him to other stuff) and Cory, who inherited the role, killed as Doc Brown. This company we have now onstage in Harry Potter and the Cursed Child, as a collective, are just fabulous. They have taken our story and made it theirs.

The show has magic and movement at its core. The company are always working. This isn’t a sit-in-the-back-and-wait-for-your-scenes type of show. It’s more: in this scene I’m pushing a staircase around; in another, I’m holding an actor in mid-air and helping him do a somersault; in another I’m a wizard at the Ministry of Magic. They sweat as one performing Steven Hoggett’s beautiful choreography and executing Jamie Harrison’s complicated magic.

We have learned from productions we have been lucky enough to have all over the world. Director John Tiffany, Sonia and Colin, and their teams have shepherded them in. John could have signed this work off to others, but together with his entire creative team, they make the show sing all over the world. We are currently performing in Japan, Germany, New York and London. On our eighth anniversary in London, we go into rehearsals for our first North American tour, with more changes required, and John is busy sharpening his pencils for that. With each new version we’re working with new actors and new audiences and this allows John to see the play anew. An illusion will get reworked, a line rethought, a costume reconsidered, and the changes rattle back through all our productions, making them all better.

Creating Harry Potter and the Cursed Child was a very beautiful and intense experience. We worked together in such tight proximity, I always joked (sort of) that it was like being back at school, but that this time some people liked me. (I know, I’m pathetic.) But it was an intensity that I never quite came to terms with. Taking Elliott to see it squared a circle that made sense of it somehow for me. The play is a play about love, and it’s made by people who love it, and Elliott made that love shine out.

• Harry Potter and the Cursed Child continues at the Palace theatre, London