When a toddler suddenly has green poop instead of the normal brown poop it's mostly normal and nothing to be worried about.
Th green poop could be caused by some foods the toddler has eaten or simply because the body didn't fully digest the food properly before it was passed through the intestines and bowels.
The bile in your intestines which is normal is green and when the food passes through the intestines too quickly it gets colored green which sometimes leads to green poop.
If the toddlers poop changes to other colors such as clay colored or pale looking then it can mean a problem with the toddlers liver.
But green poop is mostly normal and nothing to worry about especially if the green poop clears up soon but if not then it would be a good idea to have the doctor check the toddler out just to be safe.