It's not usual to vaccinate every child routinely against flu. Some children, however, are likely to have severe problems if they have flu.
Children should get flu vaccine every year (once they are 6 months or older -- we do not yet know how effective the vaccine is in younger kids) if they have:
Asthma or other chronic lung diseases (including those that stem from being a premie).
Heart disease in which the heart does not pump blood as well as it should.
Cancer or other diseases for which they are taking medicines that shut down their immune systems.
Sickle-cell anemia.
Children with other chronic diseases, such as
Diabetes
Chronic kidney disease
HIV (AIDS)
Rheumatoid arthritis or Kawasaki disease
(This is not so much because of the diseases as because children who have them often receive aspirin as part of the treatment, and we don't want to risk their getting Reye's syndrome on top of flu.)
Children (such as brothers and sisters, or kids or grandkids) who may be around people with the problems listed above -- and adults who may be in contact with such people as well. (If your child has asthma, you should get a flu shot too.
The flu vaccine is usually given as a single dose. Children getting their first flu shot generally get two shots 2-4 weeks apart; after that, only one shot is needed each year (although all of this may change in a given year depending on the latest virus drifts and shifts). It takes about a month after the vaccine is given to develop full immunity. Side effects usually include fever and aches and pains at the shot site. The outbreak of Guillain-Barre syndrome associated with the 1976-77 "swine" flu vaccine has not been seen again; Guillain-Barre is very rare among immunized people and has not been firmly connected to flu shots in kids.