Under California Law, there are several different laws relating to child sexual assault. The following are included as examples. Adding such elements as force or fear or multiple victims will lead to additional and more serious charges than those shown here.
Penal Code section 288(a). Lewd Act With a Child Under 14. The elements: The person touched the body of a child. The child was under 14 years old. The touching was done with the intent to arouse, appeal to, or to gratify the lust, passions or sexual desires of either the person or the child. A conviction for this offense per se requires registration as a sex offender. Defenses obviously include factual innocence, but also include negating the specific sexual intent outlined above. Mistake of the child's age is NOT a defense, nor is a belief - actual or mistaken - that the child consented. The touching need not be of a sexual nature.
Penal Code section 288(c). Lewd Act With a 14 or 15 year old child. The elements include exactly the same as set forth for section 288(a), with the added element that there is a ten year age difference. A conviction for this offense per se requires registration as a sex offender. The same defenses of factual innocence, and lack of the required mental state apply, as does the additional defense that the ages were closer together than ten years. Mistake of the child's age is NOT a defense, nor is a belief - actual or mistaken - that the child consented. The touching need not be of a sexual nature.
Penal Code section 647.6 Annoying or Molesting Children. This does not require touching, per se, only that the person engaged in acts or conduct directed at a minor (a child under 18) which would unhesitatingly disturb or irritate a normal person were it directed at that person and that the acts were motivated by an abnormal or unnatural sexual interest in that child. A conviction for this offense per se requires registration as a sex offender. It is not necessary that the child be annoyed or disturbed, nor is any physical contact necessary. Defenses include mistake of age, lack of sexual intent, or that the conduct wasn't proven to have been of a nature which would, without hesitation, disturb a normal person were it directed at said normal person.
THE CRIMES ABOVE REQUIRE REGISTRATION, IN LARGE PART BECAUSE THE CONDUCT WAS MOTIVATED BY SEXUAL INTEREST IN CHILDREN. CONTRAST THIS WITH THE FOLLOWING CHARGES, which might also be sought for purposes of plea bargaining to avoid registration.
Penal Code section 261.5. Unlawful Sexual Intercourse. Elements require an act of sexual intercourse (any sexual penetration constitutes engaging in an act of sexual intercourse, even if only slight penetration), that the two persons were not married to each other at the time, and that the alleged victim was under 18 years of age. There is no requirement the act was motivated by a sexual interest in children. Mistake of age is a defense. Registration is not required by law, but can be ordered at the discretion of the sentencing court.
Penal Code section 647(a). Either lewd conduct in public (engaging in a lewd act either upon one's self or another) in public or where it might be seen by members of the public OR solicitation of a lewd act (where the act was to be performed in public). Registration is not required by law, but can be ordered at the discretion of the sentencing court.
In many states child sexual assault requires a touching of a child (usually under 18) on a private spot. While it's true that there are different laws in each state that deal with sex offender classification, the federal Adam Walsh Act requires reporting under most circumstances. If there is an element of force or coercion to the sex offense and the victim is 16 or younger, I believe you always have to report federally under the Adam Walsh Act.
I don't think anyone can fairly respond that there is a "best defense for it." Every case is unique. I can say, with some confidence, that a suspect should never talk to the police when being investigated for this type of case. Assert your right to silence, don't answer questions, and speak to a lawyer in your jurisdiction.