Why does diarrhea in infant/toddler make their skin red and sore? It seems to make their skin raw no matter how much diaper/Vaseline I use.

Baby skin. Their skin is just much more sensitive to the irritation from the stools. It is a challenge when they have diarrhea to keep them from having the irritation. The only answer is to put a barrier on the skin - ointments with zinc are usually the best. Good luck.