Arthur replaced half its cast like three or four times*, and in most cases no one even noticed because, surprise, finding kids who can sound like other kids isn't hard. Unlike with adult voice actors, you're not stuck hiring only people who committed to making a full-time career out of it, of which there are only so many because there are only so many roles to go around and you need to juggle like 20 of them to make a living.
It is also the one case where one could argue it would be perfectly ethical to replace real human actors with AI. Child labor doesn't stop being child labor just because they're not choking on coal dust, and I've seen too many stories of former child actors not turning out so well. Case in point, I looked up the cast of Arthur just now and discovered that the Brain's second voice actor grew up to be the "change my mind" guy.