What will happen if a child accidentally eats a little bit of his poop?

Recycling. I'm not sure i would accept the accidental part as many toddlers believe their sense of taste is a way to understand the world. They have no social conscience at that age & may sample poo, dog food, bugs off the floor or poisons. A bit of poo is basicly just debris & germs. If it is their own, no signifant issues are likely. If from a pet, the pets gut germs may include bad ones. Watch for GI probs.
Coprophagia. Nothing! the child will be fine and his parents may be grossed out. The worst that might happen is that he might vomit. There are no other medical consequences to this very common event.