Most toddlers who have diarrhea get over the diarrhea on their own and do not need any medical attention.
However if the toddlers diarrhea lasts longer than 3 days or the toddler is getting dehydrated then take the toddler to the doctor or emergency room.
Diarrhea can sometimes lead to dehydration in the toddler if they are having too much diarrhea or are not drinking enough fluids.
You should give the toddler some Pedialyte to help replace lost electrolytes in the toddler which helps hydrate the toddlers body as well.
If the toddler is having diarrhea and is in diapers you'll need to change the toddlers diapers more often and you should also shower them off.
Even when my own toddler son was 3 years old and potty trained I put him back in a diaper when he had diarrhea as it was better for the diaper to catch the diarrhea and change him than have to clean diarrhea out of his pants and on the floor, his bed etc.
Also if your toddler is under 6 months old they should see a doctor if they are having too much diarrhea as it's easier for a 6 month old and under to dehydrate from diarrhea than it is for an older toddler.
Also if the toddler is not taking fluids or is also vomiting they should see a doctor.
Feed the toddler some more starchy foods which also help with the diarrhea and ensure they drink plenty of water.