You are correct. The statute is Cal. Fam. Code Section 17520, which states:
"(w) (1) The suspension or revocation of any driver's license, including a commercial driver's license, under this section shall not subject the licensee to vehicle impoundment pursuant to Section 14602.6 of the Vehicle Code. "

You can look up a copy of the law at http://www.leginfo.ca.gov/calaw.html . Check "Family Code," put 17520 in the keyword box, and click "search." It will bring up sections 17500-17561, so just scroll through to the section(s) you want.

You need to check on what markwj states, however I was unable to pull up any such codes that stipulate that a police office cannot impound a vehicle for non payment of child support. This is what I was able to find on the laws in California in regards to a parent that has his/her drivers licence suspended for child support. Senate Bill 1758; 30 day vehicle Impoundment for driving with a suspended or revoked drivers license. This bill also authorizes law enforcement to impound ant vehicle for 30 days which is driven by a person unable to produce a valid license upon request from a peace officer. The cost of recovering an impounded vehicle includes fines, towing fees, daily storage fees, and city administrative fees.
SB 1758; also declares it is a misdemeanor offense, penalized by a fine of up to $300.00 for any person who knowingly loans theit vehicle to an unlicensed driver.