IANALE, but it seems like someone who really didn't want anyone to have access would need to split the parts -- perhaps two lawyers who don't know about each other? One has a sealed letter to be given to the child at the appropriate age, telling them where the safe-deposit box is. Another has a package that contains the key, but doesn't know what it's the key to. A trust pays for the safe-deposit box, but the trustee doesn't have a key and isn't authorized to request one from the bank.
