Constipation is a very common problem in children. Many children normally pass stools as far apart as every few days. Regardless, you should treat hard stools that are difficult to pass and those that happen only every three days as constipation.
Breastfed infants will generally have more stools per day. Their stools vary more in frequency when compared to bottle-fed infants. For example, breastfed infants produce anywhere from 5 to 40 bowel movements per week whereas formula-fed infants have 5 to 28 bowel movements per week. Switching the type of milk or formula can also cause constipation.
Many things contribute to constipation but infants and children who get well-balanced meals typically are not constipated.
Ask your Health Visitor if a laxative might help. If it doesn't solve the problem, talk to your GP. In rare cases constipation in children and babies can be due to an underlying illness so if the problem doesn't go away in a few days it's important to talk to your GP.