Fever is a cause of concern if it lasts more than 3 days at a time and has other signs of infection or fatigue or loss of appetite. It is also of concern if it is recurrent and follows a certain pattern with any other associated symptoms. Fever is also a concern if it causes seizures with a sick looking child. Fever itself does not kill anyone, but could cause dehydration or poor oral intake due to fatigue. Fever is a cause of concern in any sick looking child.
Hope this helps.
Most often I tell people that if the fever is not preventing the patient (adult or child) from functioning - eating, drinking, sleeping - and is not causing significant distress - in the case of children this would manifest by being cranky, inconsolable - then do no suppress the temperature until it reaches closer to 103.5F.
An exception to this would be in a child with KNOWN febrile seizures, generally treating the seizure if important until they outgrow this condition which is something you should discuss with your personal physician as far as at what age you can stop avid fever suppression.