A charming and witty one-woman show with a bitter sting in its tail, Almost 10 invites us to see the world through our children's eyes. Perfectly-judged from beginning to end, it's a monologue delivered by an adult actor playing a nine-year-old girl - whose observations on life around her ring as true for us grown-ups as they would for her peers.

During her hour-long discourse, the young girl - Rachel - gives voice to much which is both funny and sad. The little cruelties of childhood are thrown into sharp relief on the bare, prop-free stage; but Rachel's a resilient young lady, and each calamity in turn is addressed with wit and verve. Above all, though, the script draws you into a pleasingly childish frame of mind, where you have permission to laugh at silly pranks and share in Rachel's mischievous joy whenever she gets one over on the grown-ups or her friends.

But I sensed all the way through this play that there was a sinister undertone; that before the hour was over, something was going to shake up Rachel's life. When the change came, it was sudden, brutally unexpected and spectacularly well-performed. Just a few moments' stillness and silence marked the switch from playful comedy to heart-wrenching tragedy - yet everyone in the audience was carried along.

What's most admirable about Caroline Horton's portrayal of Rachel is that she knows not to go too far. She doesn't, for example, use a child-like voice, something which would become annoying very quickly indeed. Instead, a little change of vocal tone or a detail of her movement across the stage is enough to remind you that you're sharing a nine-year-old's world.

And Raphaele Moussafir's script, translated from the original French, is similarly well-judged. The words in Rachel's mouth aren't realistic for such a young girl, but the play benefits from giving her thoughts a grown-up articulation. We're amused by the way she manipulates the adults around her - but we start to understand her insecurities and traumas too. And then occasionally, again, comes a logical mis-step or a delightful non-sequitur, and you're jolted back to seeing the developing brain of a child who's almost ten.

There are always a few niggling criticisms, of course. The script was generally excellent, but on isolated occasions the translation seemed flawed; at the very least, it returned over and over to some odd-sounding phrases, which became irritating through their very repetition. And I was left a bit perturbed by a scene where Rachel simulates sex between Barbie and Ken. It was funny, yes, but the question of why she understood something so adult hung over the play, and was never resolved.

Overall, though, this is a stunning script, and a stunning performance from Horton. It's a beautifully comic yet moving show, with a subtly touching message. Book it into your diary now; you'll never look at a nine-year-old in the same way again.