Poor diet will result in a weakened immune system. Try to make their eating habits healthy. Give them lot of fresh fruits especially citrus fruit and vegetables. Avoid junk foods.
Soak few almonds in water overnight(4 for your 4 yr kid and 6-7 for your 8 yr kid)
Next morning remove the skin and give them first thing in the morning followed by a cup of milk.
It build up the immunity. The kids with good immunity fell less prone to diseases.