Hello. For children 1-6 years old, pulse at rest lower than 80, for children older than 6 pulse at rest lower than 60, it's defined bradycardia. A lot of kids with sinus bradycardia don't show any symptom. If your child feels it hard to run or do sports, the problem may be more than sinus bradycardia. A slow heart beat can be simply a symptom of a more serious heart problem. I had a classmate when I was in high school, a boy, he was so tired every time he played basketball with other boys. It turned out there's a hole in his heart, and he didn't know it until he grows up. I'm not saying this happens to your kid, I'm just saying, it could be something other than bradycardia. The early you know what it is, the early your kid gets help. So why don't you get him an EKG, see what it is bothering him.