Believe it or not, there is no law that would prosecute parents for looking into their minor child's property (or messages), etc.
Also, because the photographs did not depict any full nudity, any offense that would be charged would be "Harmful Matter Sent with the Intent to Seduce a Minor." This law is found at Cal. Penal Code §§ 288.2, 313, 1170. However, because it seems as the other person sent these materials and you did not send any similar materials, you cannot be charged for the offense (since you did not commit any crime) mainly because you did not ask for the pictures, as you have mentioned.
Also, CA law regarding sex offenses have to do with sexual intercourse, sodomy or oral copulation. "Kissing" is actually not a sexual offense under CA law. What that means if you were to be charged with anything, it would most likely not be any type of rape offense because no intercourse (or similar action) occurred. A link to the actual rape law is here: http://www.leginfo.ca.gov/cgi-bin/displaycode?section=pen&group=00001-01000&file=261-269
What that means essentially, and which answers your question of what charge, if any, would occur if you were to be charged, is that the only apt charge for "kissing" would be battery. Battery is the unlawful touching of another. Battery is a misdemeanor offense punishable by jail time of up to 6 months and a fine of up to $2,000. HOWEVER, if a person "consents" to the action giving rise to the battery (i.e., consents to being kissed), then the charge would not be successful.
So really, it seems like there aren't any serious charges that would follow. And if any charge (hypothetically) follows, it would be Battery but the charge would not succeed because this person consented to the action (the kissing).
Any questions based on this? If so, please ask and I would be happy to clarify anything I've provided.
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***General information provided here is for educational and informational purposes only and does not constitute legal advice, nor should it be relied upon as such. It is always wise to consult with an attorney licensed in your jurisdiction as they would be in the best position to assist regarding your specific facts***