California courts use the "best interest" of the child standard when making child custody and visitation orders.  One of the factors a court utilizes is a child's preference for one parent over another, if the child is of sufficient intellect and maturity.  It has been my experience a child must be at least ten years old for a court to consider their preference or testimony. 

Family Code §3042 addresses the issue of a child's preference and testimony.  This code section was recently amended and states that if child is 14 years old or older and "wishes to address the court regarding custody or visitation, the child shall be permitted to do so, unless the court determines that doing so is not in the child's best interests."  This statute also provides that a child less than 14 years old may testify as well, "If the court determines that it is appropriate pursuant to the child's best interests."

In my experience courts are reluctant to have children testify and when they do there are certain safeguards that a judicial officer, i.e. judge will take.  Sometimes a judge will clear the court room of all persons other than those on the case involving the child.  Furthermore, courts may take the testimony of the child in the judge's chambers with a court reporter.  The court also has the power to control the way the child is questioned by a parent's respective attorney.  Pursuant to Evidence Code §765 "when a witness is under the age of 14…the court shall take special care to protect him or her from undue harassment or embarrassment and to restrict unnecessary repetition of questions…"

The court also has the power to obtain information from the child by alternative means, such as appointing counsel for the minor child and/or appointing an independent expert to interview the child, such as a child custody evaluator