Final answer:
The most important information to document about immunizations in a child's health history is the exact date the immunizations were received.
Step-by-step explanation:
The most important information to document about immunizations in a child's health history is the exact date the immunizations were received. This is because the timing of immunizations is crucial to ensure that the child is protected at the appropriate age. While the other options - dosage of immunizations received, occurrence of any reaction after an immunization, and practitioner who administered the immunizations - may also be important to document, the exact date of the immunizations provides the foundation for determining if the child is up-to-date and following the recommended immunization schedule.