The child actors would have already been paid, boycotting the show isn’t going to negatively affect that.
What’s far more important is the attention and subscription economy. Whatever platform is streaming it, unsubscribe from them and tell them exactly why. Don’t talk about the show. Don’t endorse watching it, don’t discuss with people how it did XYZ different from the books or whatever, fuck the whole thing off. Don’t even bother pirating it. Let it fizzle out like a fart on the wind. They’re going to pump millions into trying to get you to watch it and you’re going to spit in the face of all those executives desperate for your time by refusing to do so.
Emma Watson is older than Joanne when she first published; it’s so creepy that Joanne is desperate to infantilise her.