California Code, Penal Code - PEN § 1127f

In any criminal trial or proceeding in which a child 10 years of age or younger testifies as a witness, upon the request of a party, the court shall instruct the jury, as follows:

In evaluating the testimony of a child you should consider all of the factors surrounding the child's testimony, including the age of the child and any evidence regarding the child's level of cognitive development.  Although, because of age and level of cognitive development, a child may perform differently as a witness from an adult, that does not mean that a child is any more or less credible a witness than an adult.  You should not discount or distrust the testimony of a child solely because he or she is a child.