Kissing is fine. Anything more could be trouble. Sex is definitely out as that is statutory rape even with consent. The parents could agree verbally and then change their mind.

Can a 18 year old date a 16 year old in California?

In California, it is illegal for an adult (someone 18 or older) to have sex with a minor (someone younger than 18), even if the sex is consensual. Those who break the law have committed statutory rape.