Here's a link to the CA courts website help section. It has all the questions and answers about changing a child's last name on it.
Be sure to read and understand that changing your children's name to your husband's name does NOT make him a LEGAL PARENT of theirs. It simply gives them his name. The ONLY way to make him a LEGAL parent is for him to adopt them. And that's a whole other issue. But there's also a Help section for THAT too.