They include contact details in the letter and if there's an issue, authorities can call them - in reality, most my understanding is they generally accept the letter, esp if accompanied by a birth cert, and if the child is old enough to say 'this is my granny, we're going on holiday/ to visit cousin x for three weeks' if asked, which at 8 they will be. More and more countries refuse to let one parent travel with children without a letter from the other parent, so its fairly standard now, and many children will go away on holiday with their grandparents/schoolfriends/aunties etc.