Think low-tech. When your daughter complains about her heart, put your ear up to her chest and listen. Measure her heart rate, and listen for PVC's or other anomalies. I know all to well when at 6, I complained to my mother that my heart was feeling funny. At first, my mother dismissed it to being ill, but 24 hours later when my lips had turned blueish looking, she called the doctor. My heart rate was 312bpm.
p.s., from wikipedia "A PVC may be perceived as a skipped heart beat, a strong beat, or a feeling of suction in the chest." A lot of people will also report a pounding or pressure in their neck/throat area.
PVCs are usually benign. It depends on the structure of the heart and its muscle integrity.
The only way to know for sure would be to catch the event on an EKG.
Sometimes people will refer to feeling premature ventricular contractions (PVCs) in their throat.
Good advice above, please check with her doctor. The comfort offered above is also excellent I believe. Do worry "too" much, but do check with the doctor who know/treated her heart condition.
It is common for children to complain about their hearts; having said that, you should probably call her pediatric cardiologist and let him know about this concern. More than likely she is having premature beats and the hard 'thump' she feels is what she is calling pain. She is probably just fine but make the call anyway.