The first time a young child receives the flu vaccine the dose is divided in half and administered within two weeks to two months of each other. This is because the flu vaccine in one dose may be way too much for a young child's immune system, partly becasue it is raised in an ovalbumin rich environment and if the child is allergic to it she could be harmed from a large dose of allergen. If your child has previously gotten a flu shot then there is no need for it to be split in two.