Oh, those rascally children WISH they had so much say so in where they live.
The child can SAY what she wishes, but there are many other factors in the decision.
In order for others to get custody of her, her mother would have to be shown unfit (as would her father) and their parental rights would have to be terminated by a court.
Then the adopting couple would have to show that it was in the best interests of the child to be adopted by them, and the child's wishes would come into play at this point.
Parental rights are very strongly protected and cannot be usurped merely because a child wants to live elsewhere.
(Besides, didn't we ALL want that when we were that age?)