Since this was asked under the custody forum, is this regarding mail sent by another parent to the child? Is there mention of such a situation in the court order?
My parenting plan includes this under parents rights, and says :
When I was NCP the CP or his wife would open mail and either throw it away or use it as leverage over the kids. Example: your birthday present came today, I put it in my room and you cannot have it until I think you deserve it.. Or, the card your mom sent came today, I needed the money so I took it, but I don't know where the card is now. I always told the kids when I was sending mail or clothes/items they requested or money for book fairs/trips and the like. My grandmother used to send them money or gift certificates then stopped because they would never get it. I can remember one Halloween she sent them McDonald's gift books, dad and step mom kept them and used them, in front of the kids, while laughing and saying tell your granny it was good.
So I am biased, I think children can open their own mail when it comes to family and friends. My daughter gets mail weekly from her friends of the past.