To ice it yourself for children's cake or a class that involves kids? If you are looking for an adult decorating class check out Poon Huat - they do a Wilton Cake Decorating class... there is also a Wilton supply shop in the Adelphi Center that also offers classes and sells every single children's cake mold you can imagine. Once you get the hang of decorating - you'll never buy another cake... also check out BIY (Bake it Youself - they have a website).
If you are looking for a baking class for children - check out the Pantry at Loewen Garden.
I attended a baking course here together with my daughter.... it was a lot of fun bonding as mother and child over dough and icing !! http://www.creativeculinaire.com