A toddlers poop can sometimes smell sour due to the poop being in the toddlers intestine too long and building up bacteria as well.
The bacteria buildup in the toddlers poop can cause the toddlers poop to have a pretty sour smell to it and it's usually nothing to worry about.
Then again your toddler could have an allergy or other intolerance to something they've eaten which can cause the poop to smell sour.
If the toddlers poop only smells sour for a while and then clears up later then it's usually caused by nothing serious but if the toddlers poop continues to smell sour or more stinky than normal then I would recommend having your toddler seen by his/her pediatrician to get tested and checked out for any potential allergies or other medical conditions.
It's better to be safe than sorry so having the toddler tested would be a good idea.