I think framing the problem as "smartphones are bad" isn't as easy or effective as "certain categories of apps are bad".
One practical way to implement / enforce something like this could be similar to the movie rating system in the US: G, PG, PG-13, R, etc. "R" rated movies ban kids < 6 completely, but allow kids 7 and older to see the movie if accompanied by an adult.
Similarly, companies could sell versions of their smartphone that have parental controls enforced by default, with all app categories disabled by default, so that parents have to explicitly opt-in to the features and apps they allow kids to use.