Is there a lack of vitamin or mineral if a toddler eats their own feces?

Yes. Whenever anything is eating only its own droppings, eventually all of the nutritional elements will be drained out and it'll be as useful as eating dirt.

But children should never eat their own feces!! Human feces contain E. Coli and other bacteria. E. Coli, when consumed, causes a person to become very ill, eventually go into a coma or unconscious state, and then DIE. Hospitalization is mandatory. It's deadly.