The fact that the child qualifies you for a state or county benefit does not necessarily mean the child will also qualify as a tax dependent.
The IRS has a web site where you can determine if the child qualifies as a tax dependent.
https://www.irs.gov/help/ita/whom-may-i-claim-as-a-dependent
Generally speaking, there are two sets of dependent rules.
For a "qualifying child" dependent, the child must be your biological, adopted, step- or foster child (placed by a qualified placement agency under state law), and must live in your home more than half the year, and must not earn their own support. For the "qualifying relative" dependent, the child does not have to be related to you, but must live as a member of your household for the entire year. All dependents must be US citizens or residents and must have social security numbers.
