Usually if someone over 18 is caught smoking weed with someone under 18, they will be charged with the misdemeanor crime of contributing to the delinquency of a minor.
However lately the District Attorney’s office has been choosing to punish the exact same act with felony consequences. The same act of contributing to the delinquency of a minor can also be charged as a felony, Health and Safety code 11361 (b)(1) which is furnishing Marijuana to a minor. The code says that if someone is found to administer, furnish, sell, or offer to furnish, administer, or sell marijuana to someone under the age of 18, they will face felony charges.
Furthermore the minimum sentence to this charge is 3 years. This means smoking a small joint with your kid brother or sister could land you in jail for 3 years, and that is only the minimum sentence.