Children under age 12 on the day of departure are exempt from PMC (which is AUD 47 for tickets sold before July 1, 2012 and AUD 55 now), the airline is responsible to refund the charge to you if collected in error, this is clearly documented at http://www.customs.gov.au/site/page6068.asp

The passenger service charge varies by airport as it is actually an airport facility charge, for UA you are presumably asking about SYD. The kind-hearted private record-profit making airport authority exempts children under age 2, any older and they pay the full amount. Only way to avoid this tax is to stay in transit.