Mochi tsuki, or mochi (rice ball) pounding, is an annual New Year's tradition in many Japanese households and communities.  Special mochi rice is soaked overnight, steamed and cooked, and then pounded into a paste using an usu and kine (large bowl and mallet, respectively, pictured above).  Ozoni, a mochi soup, is the first meal many people have in the new year.  Mochi also can be made into kagami mochi, rice ball decorations (usually a stack of 2 mochi balls with a special bitter orange on top).  Mochi tsuki is a time consuming and physically strenuous activity, however its communal nature and festive atmosphere reinforce familial and community bonds.  Children may be interested to compare mochi tsuki with their own family's and culture's New Year's traditions.  

Further information on About Japan:
Osechi ryori: New Year's Food 

Additional Resources:
Kids Web Japan has a brief, informative description of the tradition "The Cutting of New Year's Rice Cakes" (kagami mochi).