Children under the age of 10 should have a cell phone because, under the right guidance, they can learn the importance of it. It shows them that you trust them with important things and it may impact their behavior towards a parent or guardian.
Another reason they should have a cell phone is that it provides a form of protection. Some parents and guardians let their children out on their own at a very young age. Without protection, the child has a higher risk of getting kidnapped or hurt.
I hope this will help you. ^^