Normally drinking milk shouldn't cause diarrhea in toddlers unless the toddler drinks too much milk then it could lead to diarrhea.
However if the toddler just drinks normal amounts of milk then develops diarrhea after drinking the milk then it could be caused by lactose intolerance.
That means that your toddler cannot drink milk without getting a stomach ache and diarrhea so if your toddler constantly develops diarrhea or complains of stomach pain after drinking milk then stop giving the toddler milk and have a doctor see the toddler to check them out and get tested for lactose intolerance.
The diarrhea from drinking milk can also sometimes be caused by lactase deficiency which develops in children over time, beginning after about age 2 when the child's body begins to develop less and less less lactase.
If the diarrhea continues a doctors visit is recommended to see what's wrong.