The definition of a Companion does not require adulthood at the time of the encounter. A child who met or saw the Prophet ﷺ during his lifetime, with the relevant religious and final-state requirements, may be counted among the Companions even if the child could not personally hear and preserve a particular report.
Two separate questions:
1. Was the encounter actually established? That concerns Companion status.
2. Was the child capable of directly hearing and remembering a specific Prophetic statement? That concerns direct transmission.
Proving the first does not prove the second. The distinction between companionship and direct audition is particularly important when studying reports transmitted without firsthand hearing.