As noted above, you never were permitted to use childs credits for "...a little more flexibility" although obviously a lot of people did.
During the latter part of 2006 Disney tightened up some on the abuse, and effective Jan 1, 2007, they actually separated the credits in the computer system. So now, when you use DDP for a TS meal, you will receive two checks and two receipts for your DDP credits -- one for the adult meals, and one for the childs meals. Everything from the placing of the order to the final accounting for the credits is now separate, and you can no longer use child's credits to pay for adult meals.
Although many servers in the past have tried to ensure that diners "maximized DDP," several CM's have reported here that they have now been threatened with disciplinary action -- up to and including termination -- if they allow guests to go outside the rules of DDP.