Originally Posted by
dandylion 
I believe that is against the law to destroy or throw away someone else's mail.....
Not for a parent opening a child's mail. They have every legal right to open, screen, or throw away what they don't want the child to receive (though of course money legally belonging to the child would have to be given to them or put aside in their name).
Of course, mail coming from the other parent could probably not be blocked without an order allowing it, but that would be a matter of interference with custody or something similar, not a federal crime.