As outlined in the General Match Requirements, for Junior U11 and Youth football matches (U12 to U16), each team is required to provide at least one Runner for each game. This person is required to act according to the rules of the game (as outlined below).
The following information has been taken from the AFLs "Laws of Australian Football 2012"
Each Team Official and Team Runner(s) shall wear a uniform or other form of identification (such as a bib or armband), as determined by the Controlling Body.
A person(s) appointed to deliver messages to the Players of a Team during a Match.
13.5 Official within Fifty-Metre Arc
Unless attending to an injured Player, no Official is permitted within the Fifty-Metre Arc during the time when a defensive Player is preparing to Kick or in the act of Kicking the football back into play after a Behind has been scored. Where an Official contravenes this Law, the opposing Team shall be awarded a Free Kick at the back line of the Centre Square, to be taken by the Player from the opposing Team who is closest to that location.
Reference Source: Laws of Australian Football 2012