Fever is nothing but a nuisance for adults, unless it rises to 39.4 C (102.92 F) or higher. But when it comes to kids, a slight rise in their body temperature, is potent enough to create serious complications, if left untreated. Generally, for kids below 5 years, 37.5 C (99.5 F) is considered as high fever, and so is 38 C (100.4 F), for kids over 5 years.