Actually children don't decide- adults do. Children's wishes are to be considered, and the weight of their wishes is a function of age and maturity. When custody motions are filed, the parents are referred to Family Court Services, an agency which provides mediation through recommending counselors to enable parents to create a parenting plan if they agree. If they do not agree, most counties are recommending, meaning the the counselor will file a report and recommendation on custody and visitation for the judge to review.
As well, the same statute providing for the child's wishes to be heard if of a sufficient age and capacity to reason- Family Code §3042 - now provides for children age 14 and older (and younger is some circumstances) to meet directly with the judge when certain requirements are met. In the several years it has been in effect, it's my experience it has created a great deal of mischief, causing courts to give more weight to a child's wishes than is warranted in making a child custody determination. Simply put, a 14 year-old doesn't have the requisite level of development, maturity or life-experience to enable the impact addressing the judge has had. Absent a allegations of abuse or neglect, as currently practiced this enabling can lead to orders with effects unanticipated by the child or the courts.