Final answer:
The maturation of the sinoatrial node is responsible for the decrease in heart rate as children age from neonates to toddlers. A child's resting heart rate decreases due to this development, demonstrating an increased efficiency in heart function.
Step-by-step explanation:
The cardiovascular change responsible for the decrease in normal heart rate as a child ages from a neonate to a toddler is c) Maturation of the sinoatrial node. As the sinoatrial node matures, it becomes more efficient in managing heartbeats, resulting in a decrease in the resting heart rate.
In addition, when discussing an increase in heart rate, it's important to note that in a healthy young adult, a heart rate above 160 bpm can lead to a decrease in cardiac output. This is due to the lessening of ventricular filling time at very high heart rates, which despite an increased heart rate can't compensate for the quick drop in stroke volume, leading to a decrease in cardiac output.
Overall, children's heart rates decrease with age due to the development of cardiac structures and functions, including the maturation of the sinoatrial node that regulates the heart rate.