They're notoriously awful in the Harry Potter films, which are hardly Pinter. Yes it was probably quite demanding for them, but that doesn't improve their performances, loads of kids can act. The ginger kid can act, because he was the one cast on his ability to act, not whether he fit a certain cuteness factor too (which is why he's now doing Theatre, while Watson is raking in money doing shite films.) Both Lohan and Culkin can act, which as Archie pointed out, isn't what they were "found out" for. Tonnes of films from the Goonies to Kick Ass have good to decent child acting in it.
Also the bar is so low for them that they'll get credit for putting in competent performances in things. "Oh look, that girl from Harry Potter has acted in a thing where she's a bit mean/slutty/american and she's not completely awful in all of it. Give her an award!" "Oh look, that guy from Harry Potter is in a play playing a tortured soul and he's sort of plausible as a human being in parts of it. Lavish him with praise!"
Acting is an incredibly high reward profession, and they'd never get close to the roles they get and fees they command now on their talent alone, without the popularity of Potter. So however much of a stress doing those films for however many years was on those poor widdle kiddies, they're still stealing a huge and lucrative living from it....They're winning!