I know a ten year old, female who has taccicardia. her heart rate has reached 200. could this be dangerous for her?

Tachycardia is..: Usually not life threatening in a child with an otherwise normal heart. They may not feel well when it happens, however. Treatment is available in the form of daily medication, or a catheter-based procedure called an ablation to try and eliminate the abnormal rhythm. I recommend her parents find a pediatric electrophysiologist in your area, who would have expertise in these issues.