You didn't say if you are the other parent who has jt. custody or if a parenting time was set forth in the order giving you jt. custody or if one parent has "primary phys custody" even though the order may be titled "joint custody" .
Depending on what the order says you may have to go modify it with an OSC Re Custody based on change of circumstances.
If you are just asking how to get food stamps thats a whole different matter. If you really have jt physical custody and the child is with you, you would think you should be able to apply for aid. That's why I wonder what else the custody order says, jt cutody language is often used to make it easier to get the parties to agree but then read fine print explaining who the child lives with etc However, dealing with county workers or the state of California, you do the math!