Each year thousands of children write letters to Santa Claus, telling him all the toys and goodies that they want for Christmas.
While the modern tradition of Christmas letters is focused solely on St. Nick, there existed a custom in the mid-20th century in which these letters were addressed to the Christ Child.
Santa Claus -- Jesus' Messenger
Francis Xaver Weiser explains the Christmas custom in his book Handbook of Christian Feasts and Customs.
According to Weiser, there is also a tradition in South America where "the children write their notes to the 'little Jesus' during the days from December 16 to 24 and put them in front of the crib, whence, they believe, angels take them to Heaven during the night."
This custom is a perfect way to "Christianize" the children's desire to write letters to Santa Claus. It helps direct children to the Christ Child, while also honoring the existence of St. Nicholas, who, being in Heaven, would be perfectly suited as an intercessor.