This is a tradition we started when Alex was 3 years. I want Alex to know who St Nicholas is and how he turned into Santa Claus over time. Santa Claus is fun, but I think too much emphasis is placed on gifts, Santa Claus and busyness during December. We want Alex to know that Christmas is not all about Santa Claus, but about the celebration of the birth of Christ.
There is lots of information on the Internet about St Nicholas. My favorite website is http://www.stnicholascenter.org/Brix?pageID=23.
This is the Russian Winter Icon of St. Nicholas.