A possible cause of a toddlers poop being black instead of the normal brown color is due to eating foods that could be black in color such as black licorice or even blueberries.
Other things that can cause a toddlers poop to be black are Iron medicines, too much iron, Pepto Bismol and even lead.
Your toddler could also have a problem with their digestive tract such as rapid or heavy bleeding that occurs in the upper GI tract can cause bright red or black stools to occur in both adults and toddlers.
Bleeding in your esophagus or even stomach can also cause you to vomit blood as well has have black or tarry stools.
If your toddler hasn't eaten any black foods and/or the toddlers poop doesn't return to a normal brown color then a doctors visit is required to find out the full and true cause of the toddlers black stool.
Having a black stool in either a toddler or adult for too long is not normal.