Nowadays, children get a phone earlier and earlier. This can have problems although some parents think that the use of mobile phones is good and their children will be safer with them. This situation is very different from the one we had some years ago when the children had got a phone when they were teenagers.
The use of the mobile phones is dangerous especially if children use it. Phones emit RF, which is very dangerous for the human brain, but they are more dangerous for a child brain because it is forming and the RF interferes in the development. This is one of the problems of the mobile phones, but there are some more, for example, it can produce cancer.
Some people think that it is good for a child to have a mobile phone just in case he or she has an accident, the child could phone the police, for example. But from my point of view it is only an excuse to give a child a mobile just for fun.
Mobile phones are very dangerous and I'd never give them to a child who is under fourteen years old. People aren't aware of the problems that they can cause because they are long-term, but a few years later the problems will be visible.