The resting heart rate of a newborn is from 120 to 160, 0 to 5 months is 90-140, 6 to 12 months is 80-140 and 1 to 3 years is 80-130 beats per minute (taken from Related Link).
To the right of the sternum, just below the ribcage is a good place. Also, directly on the sternum and just below it, around the diaphragm. You can get a general idea of an infants heart rate by...