A wallet-sized Snapper with automatic child fares on public transport. Green Snapper cards are the easiest way for children (who qualifies as a child?) to travel and always pay the correct fare, as they are pre-loaded with the child concession. This means the child concession never expires and never needs to be renewed.
If you are a frequent traveller on the train, you may wish to use a 30-day rail pass – more information can be found on these passes here.