All passengers can order off whichever menu they prefer. We have sailed twice before and my DD (now 10) has always ordered off of the adult menu. She loves to try new things and the children's menu just doesn't do it for her. DS, on the other hand, prefers his chicken fingers or hot dogs. He will try new things, and often likes them, but also loves his old stand-bys. He has told me that on our upcoming 7-day Eastern he plans to order off of the adult menu sometimes and the kids' menu sometimes.
Also, adults are free to order things from the kids' menu if they want to. I know of many adults who regularly order the Mickey ice cream bars from the kids menu...myself included!
Hope this helps. We find the cruise is a great oportunity for our kids to try new foods.