Why is my child having large, hard stools and what can I do to help?

Diet and exercise. An old computer term, "garbage in, garbage out" sort of applies. Much of the modern U.S. Diet tends to cause constipation: fried foods, cheese, white breads & pastas, white rice, red meats, sweets, bananas, excess milk, & more. Avoid or limit these foods. Instead, use fresh veggies, fruits, white meats, lots of water, whole grain cereals/breads/pastas, brown rice. Exercise/activity helps, too.
Maybe Nothing. Just because it is large and hard, there is no problem if the baby/child is suffering no discomfort at all. To improve the situation, add dietary fiber and make sure there is nothing in the diet that is causing the problem.
Stool retention/diet. It is common for kids to try avoiding stooling as soon as they have a painful experience. They do not understand how their plan hurts them in the long haul. A diet rich in vegies & fiber helps. Avoid constipators like apples, pasta & junk food. Encourage more fluid intake and regular and complete emptying. The problem often takes a year or more to fix & often needs your physicians help.