Can immediate relatives already in US teritory becomes green card holders if minor child is US citizen?

NO, the US citizen child can petition for his/her parents' permanent resident status only once the child turns 21.  Until then, the parents do not derive any immigration benefits from having a US citizen child.  However, it is possible that if the parents work for a company in a US territory that the employer can petition for their work visa and/or permanent residence on the basis of employment.  You need to provide more information about the nature of your work position to have more detailed information on whether or not you might qualify for a work visa.  Once the child turns 21, he/she can petition for the parents whether they are within the US (as long as they entered the US lawfully) or if they are still outside the US.