Fevers are the way the body kills off whatever is going on. I've been told over and over again that fevers are safe until they pass the 103 mark. Anything to bring them down inhibits the body to take care of the problem. I said that to say that it should be fine to wait for your regular physician.
There are a few ways to boost her immune system without anything invasion or anything with side effects. I give my kids Elderberry extract which is simply food but boosts the immune system so that he is stronger to fight whatever it is. Water is always good. Fever burns off moisture and the body needs to stay hydrated.
I don't think the ER will do anything but check her vitals. Your regular physician that knows her will probably do a better job. Fevers in little ones are common and it's amazing how many times they will get them with no symptoms...