An interesting book on this is the Anxious Generation by Jonathan Haidt

Smartphones are only part of the problem. Overparenting and refusing to allow children to explore the world and make their own mistakes and learn from them also significantly contributes to this. In days gone by a child would have been chucked out of the house and told to go play until tea time. Nowadays, parents never let their kids out of their site and mollycoddle them so that they never experience or learn from anything. When they eventually enter the real world they are overwhelmed.