My daughter is 7 and I know that other children at her school do have cell phones, but I do not think it is appropriate at her age. I understand the need to have contact with your child at all times, but my response to this would be why would you not have contact with your 7 year old? Where would they be that you cannot reach them?
My daughter is either at school, daycare, a family member's, or a friend's house. If they do not have a telephone or I have no means of reaching her while she is there then she would not be allowed to go.