If you cannot reach the doctor, take your child to the nearest emergency center for evaluation. In a child older than 3 months but younger than 2 years old, call the doctor or visit the hospital if the fever has persisted for longer than 24 hours.For children older than 2, a fever may require immediate evaluation if it has lasted for longer than 72 hours. temperature of 100.4 F constitutes a significant fever, according to the American Academy of Pediatrics. A rectal temperature -- taken in your child's bottom -- is most accurate if your little one is younger than 3.