There are a lot of studies that show that the phone has a negative impact on the development of the child.
You can google: why smartphones are bad for kids site:*.edu
But even without all of this researches, I see a difference between children whose parents do not control access and those who have control.
I also spoke as a parent of a child (6yo).
In our family, the child does not have access to TV, computers and smartphones.
I can now see the difference between children of his age who have endless access to telephones, gaming consoles, TV,
and with whom "old" ways of knowing the world are practiced: games, drawing, music, reading books with parents, crafts, etc.
This is very hard to do, as the parent. We live in a society in which a lot of parents
not even though to reflect on this topic and to think what will happen next with their child.
So often you can see how parents use the phone as a means of appeasing a child in any situation.
These children are no longer ways to come up with something. Their head is already full of forms, images from computer games, TV shows.
Many of them are physically weak for their age. This is scary.