ECG on my 2 y.o. son revealed polytopic supraventricular extrasystoles. Holter analyses will be performed soon. In general, his behaviour is totally normal (no symptoms of fatigue, or other of any kind). No one in family with heart issues. I understand this is little information yet, but how much such ECG is common for 2 y.o. , and how much (in %) could it be a situation to worry about?
PVC(premature supraventricular contraction) or supraventricular systole can be completely benign as in your case, or they could occur with severe symptoms. They can be an incidental finding at many times. If the child has no symptoms, just the removal of triggers like caffeine or any Chat Doctor.  If it is associated with symptoms, anti-arrhythmic are used. In absence of any structural heart defect, the condition is benign and there is little to worry about. In presence of any defect, they can cause severe arrhythmias and symptoms. Holter method will simplify things further. Also get an ECG and ECHOcardiography done for your kid to rule out structural defect of heart.