Hi thank you for posting on HCM, Low temperature in a healthy child is bit unusual as hypothermia usually occurs in a child with shock or undue coldness in external environment. Heart murmurs are very common in children and most are innocent especially if your grand daughter has grown up adequately for her age and can play normally. Therefore heart murmur cannot explain low body temperature. Are you sure about temperature measurement technique or the thermometer? If still low temperature it may be due to hypothalamus temperature regulation problem which is extremely rare in otherwise healthy child. Hope the answer is helpful. Thank you.