For example, If a child is with a friend, And their parents want to know how they are doing, You can get a burner phone. Burner phones can be purchased for under $75, That is nothing compared to a parent's wallet. When the children don't need the phone anymore, Take it away from them until they need it. Regarding needing glasses, There is nothing wrong with needing glasses at age five. Even I have been staring at screens for at least 2 hours a day, And I still don't need glasses.
I have to disagree with the fact that it is bad for eyes because it barely affects our eyes at all. It would take many, Many years of looking at a screen for it to damage our eyes enough we would notice. Furthermore, Books also strain our eyes yet kids under 10 still use books. However, There are other negative effects that it can have on young children, It can lead to them getting very distracted during important events. Another thing is that they are usually not responsible enough. Every kid under 10 I have known has broken some sort of expensive electronic device.
Kids under 10 use smart phone is bad because it can damage our eyes. If our eye is damage we can get glasses even if the kid is 1, 2, 3. . . . . . And so on and having glasses too early is very bad. So kids under 10 use smart phone is bad