Heartburn gets its name because it feels like your heart is hurting. Don't rule that out. She could have acid reflux which is very common in children. She may be in for a life of watching what she eats but I hope that's it. It's better than a life of heart issues.
Chest pain is caused by a lack of blood flow to the heart muscle. For the most part that is caused by clogged coronary arteries which doesn't happen in children. The main cause for TRUE heart related chest pain in children is from one of the cardiomyopathies or sometimes congenital heart disease where thetre is another type of structural problem with the heart. Having said all of that, true heart related chest pain in children is EXTREMELY rare, even in children with known heart disease. The fact that she says her heart feels like it stops, tends to lead me to believe she is having premature beats and that she is feeling her heart beat against the chest wall; children tend to describe palpitations as chest pain, even though it is not true heart related chest pain. It is always best to take her in, just to be on the safe side, but I don't think you really have too much to worry about. If she was having chest pain due to heart related issues, her heart would have had to be significantly damaged in order to have caused that pain and you would KNOW there was an issue going on because she would be having pain anytime she tried to do anything like playing, running around, riding her bike; she would not want to do any of those things because of the pain. Take care.
On a side note, there is a Pediatric Cardiology Forum here for parent support as well; this forum deals mainly with adult issues, just for your own info.
Take her to the doctor. It's better to have her checked out. Even children can have heart problems. It's unusual for a child to complain of chest pain; they're not that aware of internal organs at that age unless someone in the family is having heart problems and she's overheard. Please take her in.