Television commercials in particular stir up desires which would not otherwise exist.
Example: My son doesn't want a Hot Wheels until the TV commercial reminds him that he wants one. He has enough trouble with coveting, I don't need his desires further inflamed.
Television commercials, without exception, communicate one central message: Your desires are important, and must be fulfilled.
Example: Every television commercial appeals on two levels: needs and wants. These two become so intermingled that wants and needs become almost indistinguishable.
Television commercials promise fulfillment which only God can ultimately bring.
Example: I am reminded of the chocolate commercials which run every afternoon during Oprah where the woman puts the chocolate in her mouth and then she magically floats away into fantasy land.
Television commercials reinforce destructive patterns of self-control (or lack thereof)
Example: Commercials do not function in any way to communicate truth. Their function is to expand the market share of their particular product. Therefore, the undiscerning viewer knows only unfulfilled desire. Self-control and self-denial are part and parcel of the Christian approach to self, and yet they are both antithetical to the message of modern advertising.
To get polemical for a moment, it is not hard to believe that the majority of evangelicals finds Calvinism so revolting and Arminianism to be so pleasant. They have been told for the last 60 years (longer, really; we can't hang all of this on TV, after all) that they are important, that all things work together for those who will pay for it, that all emotional deficiencies can be corrected if we will only acquire the right thing for ourselves. Rejecting all of these premises are preconditions to being able to believe that God is the center, that we are fleeting mists, and that God is the most meaningful person in the universe, and yet they are drilled into our minds continually if we submit ourselves to the culture of advertising.
If we as Reformed Christians want our children to be able to learn the catechism, memorize scripture, to enjoy reading, to be able to focus, to develop a sense of self-control, to develop a healthy sense of their own sinfulness, and to see the universe as revolving around God and not around themselves, then we should understand modern advertising to be incredibly destructive to those ends. Does that mean that seeing a commercial for a Dora the Explorer backpack will ruin your daughter for eternity? No. But it does mean that the implicit assumption in the commercial for the backpack [that she should want this item, and that this want will result in emptiness if it is not filled] is never challenged. As sinners, we and our children don't need any more help being selfish. We are enough of our own enemy as it is.