Products made from the common eastern gray kangaroo and red kangaroo are generally permitted to be imported into the US, though products from threatened and endangered kangaroo species are not.
You do need to declare it to Customs. It would be helpful if the product includes documentation of what species of kangaroo it was made from, though if it was something sold as a tourist souvenir in Australia, it will likely not be something of concern.
See this FAQ entry from the Fish and Wildlife Service, shown here in relevant part:
Do I need a permit to import, export, or re-export kangaroo hides or hide products? The eastern gray kangaroo (Macropus giganteus) and red kangaroo (M. rufus) are not listed under CITES or the ESA. Other species of kangaroos, rat-kangaroos, tree kangaroos, and wallabies may be listed. Click here for a fact sheet on CITES and ESA.
- To import, export, or re-export hides or hide products from the eastern gray or red kangaroo, you do not need a CITES or ESA permit from us. You should check with the Australian wildlife authorities if importing directly from Australia.
- If you are importing kangaroo hides or products for your personal use in your accompanying baggage, you must declare the items on the Customs declaration form.
And of course I suggest not making anywhere in California your port of entry.