Whether or not grandchildren inherit from their grandparents if their parent predeceases the grandparent depends on whether the grandparent passed away with a Will or without.
In general, if a grandparent dies and their will specifies that their assets should pass to their children, but one of their children has already passed away, then that deceased child's share of the inheritance would typically pass to their own children (the deceased child's children), i.e., the grandchildren of the deceased grandparent. This is known as the "right of representation" or "per stirpes" inheritance. A will might also pass the deceased child's share to the other children, providing nothing for the grandchildren of the deceased parent.
However, if the grandparent died without a will, the inheritance would generally be distributed according to the California intestacy laws, or the laws of the state where the grandparent lived. In California, grandchildren may inherit from their grandparents pursuant to California's intestacy laws, if their parent has pre-deceased the grandparent, in a "per stirpes" inheritance. This is guided by California Code, Probate Code - PROB § 6402 and CA Prob Code § 240.
It's important to note that the laws surrounding inheritance can be complex and vary depending on the specific circumstances involved. It's always a good idea to consult with an estate planning attorney to ensure that you understand your rights and obligations under the relevant laws.