Can herbal supplements that come from china such as kava kava or valerian root have benzos or other substance that are not listed on the bottle ?

The two: Herbal supplements you listed do not contain benzodiazapines. However, i can not vouch for quality control for whatever company sells the supplements in question.

Yes: Herbal medications are not generally regulated and tested as are prescription drugs in the United States, so it is certainly possible for other substances to be present. There are reputable makers of many herbal preparations, such as those sold at GNC and most chain drug stores, but others, esp available online, may not be adequately tested. Benzos have been reported to be in some of them.