The safest and most effective pace for your heart to be beating during exercise is at 50-85 percent of its maximum capacity. This calculator provides you with an approximate target range for your heart rate during exercise, based on this recommendation.
Target Heart Rate Calculator
How to measure your resting heart rate:
This tool is not applicable to children; the maximum heart rate in healthy children is about 200 beats per minute, and unless there is just medical cause, there is no need to arbitarily limit the heart rate of a healthy child.