Diarrhea usually isn't cause for concern in toddlers, children and adults until the diarrhea lasts longer than 3 to 4 days.
If the diarrhea doesn't stop after 3 to 4 days or seems to get worse or the child starts vomiting or feeling more stomach pain then an urgent care visit or pediatrician visit is recommended.
The pediatrician may have to prescribe the child some medicine for diarrhea and if the diarrhea doesn't go away after a couple days then I would take the child to a pediatrician just to be safe.
Sometimes diarrhea is caused by eating too much fiber or drinking too much milk but the diarrhea can also be caused by stomach flu, and many other viruses that are not very severe but other viruses and bacteria can be more severe.