Useful reframe: it's not old vs. new tech, it's tools you command vs. media that commands you. "Retro" correlates with "good for kids" mostly because old tools aren't engagement-optimized — they sit there until the kid acts. A modern non-algorithmic tool can be just as good.
What a dumbphone doesn't solve is the social tax — opting a kid out of the addictive layer can also opt them out of the group chat. That's the actually-hard part.