You can't use children's credits to buy adult meals unless the restaurant has no child's menu. There are a few CS (not many) that have no child's menu.
As far as adults purchasing children's meals there are a few things you can do:
1) Switch meals with a child. This will work at either TS or CS. What they care about at a TS is that a child's credit is used to order from the child's menu. Once the meal arrives, the adult can eat the child's meal and the child can eat the adult meal.
2) Use your adult dining credit to purchase a child's menu meal, assuming the restaurant will allow you to do this. Some restaurants, apparently randomly depending on what server you get, may not allow an adult to order from a child's menu. Make sure you specify to the server or cashier that you want a child's meal but are spending an adult's credit.