Hello, Thanks for posting your query. Your child if diagnosed as having lactose intolerance needs to be thoroughly evaluated by her pediatrician and nutritionist and a personalised diet plan needs to be followed excluding milk and milk products. The vomiting and diarrhoea occur due to the allergic reaction of her gastrointestinal tract to the allergen ( lactose ) in any dairy product. So the best way to prevent is avoiding the allergen. Apart from that temporary relief can be obtained by antiemetic ( domeperidone) for vomiting and probiotics and multivitamin - zinc supplements to decrease diarrhoea . Most importantly maintain her hydration with round the clock oral rehydration solution( WHO ORS) during such episodes to prevent dehydration and weakness. Wishing you and your family good health. Regards