Acidic diarrhea in toddlers is usually caused by acidic food that the toddler has been eating.
The acidic diarrhea should clear up once you change the toddlers diet to non acidic foods but if not it would be best to speak with the toddlers doctor about the acidic diarrhea.
Acidic Foods are foods such as foods containing grains, sugar, fish, some dairy products, vegetables meats and fruits and much more.
The diarrhea itself may not be caused by those foods but instead may just be a viral infection that should go away and the stools are just acidic and should go away on it's own within a week or two.
If the toddler continues to have diarrhea and it's still acidic then I would make an appointment with the toddlers pediatrician to make sure everything is okay.
But the acidic stools in toddlers as well as adults is usually caused by the types of foods that you eat which just affects some people more than others.
The toddler could also have Malabsorption which is a condition that prevents the absorption of nutrients through the small intestines so it would be a good idea to have the toddler checked out especially if the acidic stools or acidic diarrhea doesn't go away soon.