Iron is a nutrient that's essential to your child's growth and development. Iron helps move oxygen from the lungs to the rest of the body and helps muscles store and use oxygen.If your child's diet lacks iron, he or she might develop a condition called iron deficiency.ron helps move oxygen from the lungs to the rest of the body and helps muscles store and use oxygen. If your child's diet lacks iron, he or she might develop a condition called iron deficiency.