Make that 312 bpm as a child...
It sounds like some variety of tachycardia, perhaps SVT. Whether it's serious or not would require some more monitoring. Did the monitor catch anything? Oft times, a 24hr. Holter is useless because of the nature of SVT. A two lead, 30 day monitor works best for catching this kind of stuff, and is easily worn by the user. I had 54 years of SVT before getting it fixed 4 years ago. I had rates that averaged around 220 bpm., but was clocked as high as 31pm in the hospital as a child.
Sounds like serious tachycardia ? Self terminating ? Has she seen a cardiologist and has her QT interval been checked