Boosting My Kids Immunity?

I have two kids aged 2 and 4 who have been struck down with lots of colds and flu this winter. They have also had boils. They are on antibiotics but apparently these dont work on viral infections. I want to boost the kids immunity but am unsure how. I have bought the 'yaukult' drink for them, but dont know if this is very good for kids. Are supplements any good at boosting immunity?