Exercise increases the heart rate (HR) to circulate more blood to the lungs and thus increase oxygen due to more energy required by the tissues. Generally, the maximum heart HR is 220 minus age (in years), children have a higher HR, for instance: a 5-year-old can rise 215 bpm (220-5 yr = 215 bpm) when running at high intensity.
if the children present chest pain or chest discomfort when running you should be concern about, in that case your pediatrician will should assess the need for a review by a specialized cardiologist.
There are situations that make the heart go faster such as anemia (as there is less hemoglobin that transports oxygen, the heart compensates by increasing the heart rate) or hormonal alterations, among them hyperthyroidism, that when there is elevation of this hormone the heart goes faster, and conversely, in hypothyroidism the heart goes slower.
If there is any doubt in a child with palpitations, assessment by pediatrician should be performed who decide if blood analysis will be performed to rule out extracardiac abnormalities.