First, let me congratulate you for your interest in having your child learn or maintain another language.  There are many academic and social advantages to being bilingual or multilingual in this world.
Every school district has an administrator that is in charge of "foreign languages, Services for English learners, Bilingual programs, or Categorical programs".  My advise would be to call Modesto Unified School District and ask for the office and person in charge.  They will be able to tell you what is available in the school district.  If they do not offer classes to learn Spanish, another possibility would be to hire a private tutor - such as a high school student or college student who are Spanish native speakers or majors.  You can also think about a community college, or private schools.
Thank you and good luck.