I think Disney tries to have kids' menus that offer what kids like to eat. For years my nephew only liked grilled cheese or chicken nuggets. He was happy at WDW because he could get whichever he was in the mood for. The last time he went, he was 15. He still wanted chicken nuggets. He had no problem ordering off the kids' menu -- we simply ordered him two kid meals instead of one!
Our twin boys would eat mac & cheese for every meal if we let them.
If a 6-year-old looks over the menu and orders the $30 cedar plank salmon, that child is in the minority of children in my experience, and I question whether there's any parent on the planet that is going to buy a 6-year-old a $30 entree.
If your child is the rare 6-year-old that orders the cedar plank salmon and you know that, can't you buy the adult
DDP for her instead of the child's DDP?