Chances are the ones in your family that have had the flu now have a natural immunity (which is typically stronger than gaining immunity through a shot) and won't get it again.
If you want to boost their immune systems, Kiddieboost looks good, but expensive. I have used Floradix kiddie vitamins which is around the same price. I would head towards a Whole Foods or other HFS (Health Foods Store) and see what they recommend. You can get echinacea (cone flower)in other forms for much less. We have an echinacea/goldenseal tincture that we use from Earthfare, large bottle is about $11.
Research has shown that Vitamins C & D are very important to immune health. A LOT of people are Viatmin D deficient.
http://www.scientificamerican.com/blog/60-second-science/...
In addition to a multi-vitamin right now we supplement with a Vit C. & Vit. D. So far so good. But in all honesty, with just following the rules of good nutrition (cut out refined sugars/processed foods & cereals) you can maintain your bodies immune function. Lots of fruits and veg, and lots of leafy greens.