Actually, the kids meals really are that cheap. That's really got to be part of the problem. If the difference in price between regular meals and child meals wasn't so large, then (1) adults wouldn't be tempted to order child meals, and/or (2) the restaurants' revenues wouldn't be so adversely affected when they did.Plus anything at Disney isnt really that cheap, even the kids meals.
Regarding the difference in price, I've mentioned this several times before. The cost of the food is a small part of the costs associated with your meal, covered by the price you pay. Besides contribution to profit, there are labor costs, equipment costs, facility costs, and the cost of customer acquisition. These collectively dwarf the cost of food ingredients, and they are not significantly affected by the size of your meal... whether you're an adult or child, your chair, for example, "costs" the same.
What ends up to be the case, most of the time, is that many young families are deterred when they see a "fair" price for a smaller portion of an adult meal. There are places Disney offers it (especially at the lower-priced TS like Beaches and Cream) but where young families would be substantially deterred, Disney offers a standard offering, because that's what we customers (not any one of you individually, but us all as a group) is driving Disney to do.