My children did not see commercials yet but they love to make fakes ones!
But what is a commercial: it is a Television advertisement.
"The vast majority of television advertisements today consist of brief advertising spots, ranging in length from a few seconds to several minutes (as well as program-length infomercials). Advertisements of this sort have been used to sell every product imaginable over the years, from household products to goods and services, to political campaigns. The effect of television advertisements upon the viewing public has been so successful and so pervasive that it is considered impossible for a politician to wage a successful election campaign, in the United States, without use of television advertising." (http://www.answers.com/topic/television-advertisement)
This is exactly what I explained to my 6 years old children: commercials exist to make you do what they want instead of what you want, they exist to create needs that you would never have without them. And for all that, they lie. They got it and it makes them laugh, so they love making some fake ones.
What is the idea in all that: making is taking control. Knowing, understanding what a commercial is, how it's made and for what goal, will build a kind of protection against its influence.
This is part of this media education I believe essential to give to our children today, in this world where Television has such a big part of the life of many families.