I think it is okay to give a child a phone so long as there is proper guidance and restriction. Having a mobile phone is one of the easiest ways of communicating with them, especially when you are away from home. Imagine the worry, and the thoughts that would come to mind, had you not been able to check them from time to time.
However, children should always be guided on their usage, as there are various content on the internet that are not suited for them. Before handing them a phone, it is best to explain, advice, and teach them the dos and don'ts of using mobile phones. Moreover, as children tend to be addicted on new stuff that catches their attention, I also think that parents should limit their phone usage and make them understand that actual interaction with people is always better.