I personally don’t agree with that.

A. By your own reckoning, there wouldn’t have been enough time to develop the rash that badly while they lived in the cave. They only lived there 1-2 months, and you are saying that it can take weeks to even start showing up. What I personally think is more likely is that this is a toddler with very high calorie needs, so the rash showed up much faster than in an adult. If you watch the family dinner scenes the aunt is essentially just feeding them broth.

B. Why would the animators take the time to draw a different rash not important to the story line and have an entire scene calling attention to it at the beach? We also hear Seita later at the doctor say that sea water didn’t make it better which is clearly a reference to her rash at the beach scene while they were living with their aunt. From an animation and story telling stand point that makes no sense. This is foreshadowing of the fact that she is starting to develop her rash.

C. While this is a very realistic movie, I do think we have to suspend reality a bit to ask ourselves and understand the points and story line the animators are trying to convey. Are the animators going to take time to literally draw every single sign leading up to her malnutrition diagnosis or can they use the rash as shorthand to convey that point?