Part of my answer to this question may be a dream for a far away land. I think that a kid should not have adult toys. Children do not need cell phones, TVs, video games, or any other sort of electronic babysitter.
Children need the attention, love and guidance from their parents or legal guardians. The advanced technology is creating a cyber-human society. What are we going to do when machines take over the human brain? Who is going to babysit us then? Who is going to protect us from a supercomputer that will be able to control our every move?
I can see that more and more society relies on technology to imagine that children are safer. I think is quite the contrary. With a cell phone, a child has the freedom to go anywhere after school. If both parents work, which is not uncommon, the child can be anywhere but at home when the child should be home studying or doing house chores. The cell phone facilittes mobility and the chil'd opportunity to conceal his or her real location at any given time.
I think the cell phone should be replaced by a well monitoring parent, by high standards and effective implementation of any broken rules.