Dengue is one disease in which patient feels more sick after fever gets resolved. Main worry is not falling platelets but dehydration. Dehydration occurs because of leaking of plasma from blood vessels. So it is dehydration without Diarrhea.
Main treatment is prevention of dehydration. Keep children hydrated with oral liquids, if they don't tolerate , give them Drip.
Take care that child should pass urine every one to three hours.
Give oral fluids of their choice.
Main reason for deterioration is under- hydration.