Pediatrician here- really for a healthy 6 year old I tell parents that if their child is not lethargic (hard to wake up all the way, like a limp doll), not having trouble breathing, staying hydrated and not having other concerning symptoms you don't need the ER until a fever is over 105. Plenty of run of the mill viruses cause fevers in the 104 range in children. It's not always the number of the fever but how the child looks otherwise. Tylenol and Motrin generally only bring fevers down a few degrees so a high fever won't return to "normal" with medicine but it will bring it down to make the child more comfortable. If a child is truly lethargic or having trouble breathing I don't care how high the fever is it warrants and ER visit. Fevers for the most part are nothing to be afraid of and are the body's way of helping to fight the infection. A visit or call to your pediatrician's office is completely appropriate when you are worried about a fever or it is lasting more than a few days.

Babies under 6 weeks old, however, need to go to the ER for any fever over 100.4 and babies older than that need to be seen at least in the office with a fever because their immune systems aren't all the way developed and they haven't had all of their vaccinations yet.