It is recommended to treat a fissure in a baby's anus caused by constipation by addressing the constipation itself. You can give the baby Lax-A-Day regularly, add fiber and liquids to their diet, and gradually increase their fiber and liquid intake. You can also apply petroleum jelly or zinc oxide to the baby's anus to make bowel movements less painful. If the constipation persists despite these measures, if there is persistent blood in the stool, severe abdominal pain, or vomiting, it is advisable to consult a doctor.