The general consensus seems to be if you are paying OOP and not on the DDP, older kids can usually order from the childrens menu. However there are a few reports of some servers who are not allowing the older kids to order a kids meal. The menus do state they are for ages 3-9 so it seems like there may be a few overzealous servers who are taking that very literally. But overall most reports seem to be that older kids can order kids meals. If you are on the DDP, the reports are more mixed. Some places will serve a larger portion of a kids meal and you still get charged the adult credit. Some won't allow a child over 9 to order from the kids menu since they are considered an adult on the DDP. You would think they wouldn't care since the kids meals are so cheap and you are paying with an adult credit.
At buffets kids over 9 will be charged the adult price, regardless of how much they eat.