My son had that foul smelling poop when he was a toddler and when I took him to the doctor the doctor said that when toddlers have foul smelling poop it's most commonly caused by fat malabsorption which is what it turned out to be with my son when he was a toddler.
Another medical term for fat malabsorption is steatorrhea.
It's a condition that prevents the absorption of nutrients through the toddlers small intestine.
So if your toddler is having foul smelling poop that is not clearing up then you should take the toddler to the doctor to see if that is what your toddler has.
The child's doctor will usually draw some blood and take a stool sample to test for the Malabsorption and then the treatment will usually involve a diet change.
Treatment for the toddlers foul smelling poop or malabsorption varies by disorder, however the treatment often involves diet changes.
Children who have trouble digesting lactose or sugar will need to avoid those foods or take oral medications to replace necessary digestive enzymes.
Children with celiac disease should avoid gluten in their diet and in some cases, such as with short bowel syndrome, children might require nutritional supplementation through an IV solution.
The toddler certainly needs medical attention from the doctor to treat the malabsorption so schedule an appointment with the toddlers doctor.