To convert pounds to kilograms, you can use the conversion factor: 1 pound is approximately equal to 0.453592 kilograms.
First, convert 55 pounds to kilograms:
\[ 55 \text{ pounds} \times 0.453592 \text{ kg/pound} \approx 24.947 \text{ kg} \]
Now, we'll round this to 25 kg for simplicity.
Next, calculate the prescribed dosage of chloral hydrate:
\[ \text{Dosage} = 25 \text{ mg/kg} \times 24.947 \text{ kg} \approx 623.675 \text{ mg} \]
Thus, the prescribed dosage is approximately 624 mg when rounded to the nearest milligram.