This is a lovely article about letters from Santa Claus. For many years, on behalf of Santa Claus, the employees
of Canada Post have been sending letters to children. Now they send them to classes. This article published by the Canadian Philatelist is one that a person would like to keep. There are samples of the letters. There is even an address for Santa Claus - HOH OHO.
Children are unlikely to write thank you notes but they might write a letter to Santa.