Heart murmur's are not an object inside the heart, they are a sound heard by a stethoscope that is abnormal.
Some heart murmurs
are normal and others can indicate a defect of the heart. Ask the doctor who found the murmur as to whether it has the characteristics to possibly be a heart problem. If it does, make an appointment with a Pediatric Cardiologist in your area. I recommend setting an appointment with one at an actual children's hospital.
The heart is around the size of their fist.