Thank you for your question.
My opinion is that depositions are probably not necessary if the child wants to testify.
Cal. Family Code
3042. (a) If a child is of sufficient age and capacity to reason so as to form an intelligent preference as to custody or visitation, the court shall consider, and give due weight to, the wishes of the child in making an order granting or modifying custody or visitation.
(b) In addition to the requirements of subdivision (b) of Section 765 of the Evidence Code, the court shall control the examination of a child witness so as to protect the best interests of the child.
(c) If the child is 14 years of age 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. In that case, the court shall state its reasons for that finding on the record.
(d) Nothing in this section shall be interpreted to prevent a child who is less than 14 years of age from addressing the court regarding custody or visitation, if the court determines that is appropriate pursuant to the child's best interests.
(e) If the court precludes the calling of any child as a witness, the court shall provide alternative means of obtaining input from the child and other information regarding the child's preferences.
(f) To assist the court in determining whether the child wishes to express his or her preference or to provide other input regarding custody or visitation to the court, a minor's counsel, an evaluator, an investigator, or a mediator who provides recommendations to the judge pursuant to Section 3183 shall indicate to the judge that the child wishes to address the court, or the judge may make that inquiry in the absence of that request. A party or a party's attorney may also indicate to the judge that the child wishes to address the court or judge.
(g) Nothing in this section shall be construed to require the child to express to the court his or her preference or to provide other input regarding custody or visitation. (h) The Judicial Council shall, no later than January 1, 2012, promulgate a rule of court establishing procedures for the examination of a child witness, and include guidelines on methods other than direct testimony for obtaining information or other input from the child regarding custody or visitation. (i) The changes made to subdivisions (a) to (g), inclusive, by the act adding this subdivision shall become operative on January 1, 2012.
Depositions cannot hurt in helping you prepare for the hearing but they are expensive. You can often get the same information by serving interrogatories.
I hope this information is helpful.