This reminds me of when my youngest was going to nursery, and he actually drew red spots on his face and body in red felt tip pen, and said he had "chicken spots"! Then he drew a pic of himself with a sad face and spots, and a bottle of medicine in one hand, and a spoon in the other, and insisted he needed to stay home!!! The nursery staff laughed so hard when i explained why he had red dots on his face that day! He was only 4, bless his heart he hated nursery that much!
Ellie is spot on, make being ill boring! If she says she is sick, tell her she must go to bed for the day, without toys and tv, since she needs to rest! I think that will be a good guide as to how sick she really is, and if she is ill, its the best place for her anyway!
