(This response simulates a long, detailed audio output. Since I cannot actually generate audio, I will describe the characteristics of the sound effect that a Heart Beat Sound Effect Generator would produce based on the prompt.)

The generated heart beat sound effect would feature a rapid, irregular rhythm indicative of a five-year-old boy's heart rate immediately following a run. The sound would be characterized by:

• High Frequency: The individual beats would be relatively high-pitched, reflecting the smaller size of a child's heart and the faster rate. This would not be a deep, resonant thump, but rather a quicker, lighter sound.

• Irregularity: The intervals between beats would not be perfectly consistent. There would be slight variations in timing, mimicking the natural irregularity of a heart beating rapidly after exertion. This would avoid a monotonous, artificial sound.

• Dynamic Range: The volume of individual beats would fluctuate slightly, adding to the realism. Some beats might be slightly louder than others, reflecting the natural variations in cardiac output.

• Short Duration: The overall duration of the effect would be relatively short, reflecting the temporary nature of a rapid heart rate after a brief run. The sound would gradually slow and become more regular as the simulated exertion subsides.

• Absence of Extraneous Sounds: The effect would be clean and focused on the heart beat itself, without added background noise or other distracting elements. This ensures clarity and focus on the intended effect.

• Realistic Timbre: The overall tone would be consistent with the sound of a child's heartbeat, avoiding any overly mechanical or artificial qualities. The sound would be slightly softer and less powerful than an adult's heartbeat.

• Potential for Subtle Variations: A sophisticated generator might offer subtle variations in the sound, allowing for a degree of randomness to further enhance realism. This could include minor changes in pitch or volume that would not be noticeable individually but would contribute to a more natural overall sound.

The generated sound would be suitable for use in various media, such as animation, video games, or educational materials, to realistically depict a child's elevated heart rate after physical activity. The specific parameters (tempo, volume, irregularity, etc.) would be subtly adjusted within the generator to accurately reflect the physiological reality of a five-year-old's post-run heart rate.