A healthy diet does wonders.
Fatty Acids (in veggie oils, nuts and oily fish) are one of the few nutritional needs our bodies can't produce on their own. They build good immune systems, strengthen muscles, increase energy and lower the chance of infections.
Of course fruit and veg play the ultimate role in any diet. Limit salt, bad fats and sugar. Apparently sugar can lower the effectiveness of the immune system by up to 50%!!
Here's what I found for RDA for salt intake:
7 to 12 months: 1 g per day
1 to 3 years: 2 g per day
4 to 6 years: 3 g per day
7 to 10 years: 5 g per day
11 to 14 years: 6 g per day
Remember that young kids can have up to 12 colds a year while their immune system is building itself up. I think the average for a kid before starting school is about 8 and increases when they start school.
It might seem a little strange but exposure. The more colds and little illnesses William catches and fends off now, the stronger his immune system will be as he gets older.
"The worst feature of a new baby is it's mother's singing." - Kim Hubbard