What will happen to an 11 month old if he or she receive a 36 month old dosage of the flu shot?

Local irritation: The only difference between the two is the dosage of the vaccine given - a <3 year old gets a 0.25ml dose and the >3 year old a 0.5ml dose. It's the same vaccine, though, not made any different. Your younger child may get more of a local reaction/irritation at the vaccine site and may be a bit more likely to run fever after vaccine, but other than those, it will not hurt them in the long run.