I think it's not necessary at all when the kid is too young. What you would want to do is to keep a better eye on your kid, or ask him/her to memorize all the pertinent information whenever it will be needed, like telephone number and address.
While cellphones can be a great tool for communication, it can also be a bait especially for unscrupulous persons.
Personally, I won't give my child his own phone probably until he finishes elementary.