Boy (Susan and the Daleks)

A boy once walked past the Doctor's TARDIS when Susan had parked it hurriedly as she escaped Dalek pursuers. Seeing through the open door that it was dimensionally transcendental, the boy walked in and began playing with the TARDIS control console; Susan revealed herself to him and, with a smile, began to actually explain things to her. They were interrupted by the Blue Chrono-Dalek, who caught up with them and followed the boy into the TARDIS. (TV: Susan and the Daleks)

Behind the scenes
Josh Maguire played a generic young boy in several of the skits in (More than) 30 Years in the TARDIS, including one in the pre-credits sequence.