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.
I know for a fact that a very, very high dose of Dexedrine (dextro-amphetamine) causes pupils to dilate. Dex is the active ingredient in Adderall. Everyone responds so differently to meds that itt wouldn't surprise me if your dilation is Adderall related at therapeutic doses.