Answer: probably.
http://www.foodrevolution.org/slavery_chocolate.htmThat's a bit out of date, but according to my research the planned elimination of child slavery by 2005 wasn't met, so it was extended to 2008. That wasn't met either, so it was extended further out.
This sort of goes with a thread I posted about a Chinese guy committing suicide, in that the prices we pay for things are totally unrealistic, and greatly depend on the abuse of other cultures.
As for companies that most likely support child slavery by buying their cocoa from suppliers that use child slaves:
The $13 billion U.S. chocolate industry is heavily dominated by just two firms - Hershey's and M&M Mars - who control two-thirds of the market. Unfortunately, both of these companies fall into the category of those companies who use large amounts of Ivory Coast cocoa, and whose products are almost certainly produced in part by slavery.
[...]
M&M Mars and Hershey Foods Corp. are not alone. Other companies whose chocolate is almost certainly tainted with child slavery include: ADM Cocoa, Ben & Jerry's, Cadbury Ltd., Chocolates by Bernard Callebaut, Fowler's Chocolate, Godiva, Guittard Chocolate Company, Kraft, Nestle, See's Candies, The Chocolate Vault, and Toblerone. While most of these companies have issued condemnations of slavery, and expressed a great deal of moral outrage that it exists in the industry, they each have acknowledged that they use Ivory Coast cocoa and so have no grounds to ensure consumers that their products are slavery-free.
Companies like Mars, Hershey, and Nestle often say that there is no way they can control the labor practices of their suppliers. But there are other chocolate companies who manage to do so, and it would seem that if the bigger companies really wanted to reform problems in the supply chain, they have the power and ability to do so.
There are in fact many chocolate companies who only use cocoa that has definitively not been produced with slave labor. These companies include Clif Bar, Cloud Nine, Dagoba Organic Chocolate, Denman Island Chocolate, Gardners Candies, Green and Black's, Kailua Candy Company, Koppers Chocolate, L.A. Burdick Chocolates, Montezuma's Chocolates, Newman's Own Organics, Omanhene Cocoa Bean Company, Rapunzel Pure Organics, and The Endangered Species Chocolate Company.
Personally, when I do eat chocolate (which is rare), I'll buy Fair Trade. It's significantly more expensive ($4 - $5 for a bar), but it's worth it in my mind. They talk about Fair Trade in that story, too.