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.
There is an antidilation drug, dapiprazole, that blocks stimulation of the muscle that widens the iris. In a 1994 clinical study, published in the journal Optometry and Vision Science, the drug was tested in one eye in 30 subjects who had been given drops containing the dilation drug tropicamide.