To my knowledge, you can NOT order a childs size portion of an adult menu entree at any restaurant, anywhere, not just WDW. The restaurant publishes a childs menu of items they feel they can prepare in a smaller portion. I wouldn't expect them to "downsize" adult entrees.
You can share if you like, but the portion would not be large enough for us, average eaters. If that doesn't work, you could look for somewhere else that might work for you.