Whether or not your girlfriend and future child should be covered by Medi-Cal or your health insurance is a question for those two organizations. They are not tax questions.
To get a tax benefit for them, they need to be your dependents. This is independent of how they are insured.
Note that your girlfriend may qualify to be a "qualifying relative" while your child may qualify to be a "qualifying child".
Please review the information about Dependents starting on page 25 of IRS Pub 17.
P.S. does your girlfriend have a Social Security number or ITIN? She will need it to appear on your tax return in 2020.
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