If you read here you know that the state of children's counter service meal credits is a confusing, and sometimes contentious, topic. Just a few months ago I researched it thoroughly as possible and found that Disney does not differentiate from adult and child counter service meals, meaning you can technically purchase an adult meal with a child's credit. This was confirmed up the chain of command by Disney on several occasions.
As of right now, you can still use a childs' credit for an adult meal. We've gotten reports (Update: As recently as December 2011) that guests are still doing this, but we've also heard of of the rare instance of people being stopped at some locations. Bottom line: Do what you feel works. You're not cheating the system because Disney could easily differentiate between adult and kids credits.
As soon as I hear of any changes, I'll let you know.
I'll keep you posted.
Read more about the counter-service kids meal brouhaha here and here. UPDATE: Please see this post (March 11, 2012).