It is usually from several things.

1. food sensitivities/allergies

2. Too much sugar/starch in the diet which causes pathogenic imbalance (usually fungal)

3. Speaking of fungal, mold toxicity is often a trigger

4. poor liver/kidney function and imbalance in the immune system (usually Th2 dominance)

Each child will need a different protocol so i cant give you a protocol however a good starting point is removing all histamine-forming foods form the diet especially gluten, dairy, sugar, nightshades, eggs, peanuts and yeast. You also want to get them on probiotics (containing lactobacillus acidophilus), chlorella, liver and kidney support, antifungals, and make sure all pollutants are removed form the home (skin care products, shampoos, bad detergents, etc).