A calendar offering a piece of chocolate each day through December is one way to countdown to the 25th. But why not include a 'countdown' which helps children prepare spiritually for the season? Here are some ideas you might consider.
Buy an Advent Candle which has 24 (or 25) sections marked on it. Each day light it and burn one section. (You might do this before a meal or as part of a bedtime routine.) Say a simple prayer such as, 'Lord God, as we light this candle help us to get ready for Jesus, who came to be the Light of the World.'
Read an Advent book. Both The Advent Calendar by Steve Croft and The Christmas Mystery by Jostein Gaarder have 24 chapters which take you on an exciting journey back to Bethlehem for the first Christmas.
Paperless Christmas is an on-line Advent Countdown which has a contempory take on the Christmas Story with a short video clip each day.