I agree but it is much harder to stop people from building slot machines than to stop children from using them. Even easier than telling people to stop using them.

And hey, maybe if we actually take some autonomy and remove that market from actors who don't want to build the things the market is requesting then they'll actually build the things the market is requesting... it's easy to say we want something but no one listens when we still buy the thing we say we hate. Maybe it's addiction but it's still hard to fight against. (Though we could still do better by accommodating those who are trying to break the network effects. You can complain how hard it is to get off Facebook but if you're not going to make the minuscule extra effort to accommodate those who do leave then how can you expect the ground to be laid for you to?)

At the end of the day though, with kids, the OP's argument fails because it either assumes smartphones are an inevitability or that the benefits outweigh the costs. It's a bad argument.