Many children in this age range are very aware of their bodily functions and often report things about their heart beat. The most common cause of slowing of the heart rate in a healthy child is linked to breathing and high vagal tone, called sinus arrhythmia. This is not a bad rhythm. The heart rate slows a bit during expiration or breath holding (as in crying) and speeds up with breathing. It should not be associated with chest pain. If she is not having shortness of breath at the time, and her coloring is good, then this probably is benign. If she is not looking normal, then she should be evaluated by your doctor. It might be helpful to learn to take her pulse rate at these times; your doctor will appreciate having some numbers of her heart rate at the time of her symptoms, should she continue to complain.
We can't thank you enough for your blessed insight. We will take her to her primary but have much more peace in our hearts thanks to your fast response. Thank you, thank you, thank you!