POLICY: Persons under the age of 18 are not allowed to move goals or be nearby when goals are being moved. All goal movement and placement must be done by adults.
POLICY: Goals must be anchored or games shall not be played.
From the AYSO Safe Haven Certification Manual for 2010-2011, pg 29:
The number one safety concern in soccer is the safety of goalposts. Movable soccer goals can tip over causing injury or death. This typical tip-over incident can happen when a child or young adult climbs on or hangs from the crossbar of a soccer goal which was left unattended or improperly anchored. The heavy goal tips over and injures the individual, sometimes causing death. Even a gust of wind can topple an unanchored goal.
To help avoid these incidents, the U.S. Consumer Product Safety Commission (CPSC) urges consumers, school officials, soccer coaches, players, and organizers to follow these safety guidelines:
Where to place goals
See this link.