Dehydration is one of the most worrisome complications of diarrhea in children. Mild diarrhea usually doesn't cause significant fluid loss, but moderate or severe diarrhea can. Severe dehydration is dangerous; it can cause seizures, brain damage, even death. Know the signs of dehydration.
Cough and cold medications and decongestants may cause the eyes to dilate. In general, any medication that is a stimulant may cause the eyes to dilate. While it's not particularly common, this could include ADHD medications such as Adderall or Ritalin. Of course, certain illegal drugs do this as well, such as cocaine.
How does it work, anyway? The drops we use to dilate your eyes are actually a combination of two drops: one paralyzes the muscle that contracts to make the pupil smaller (much less scary than it sounds), and the other stimulates the muscle that has the opposite effect on the iris, widening the pupil.