Payments from the state are not considered as support from the child, so they do not enter into the calculation for the support test. As long as the child does not provide over half their support, the foster parents can claim them as a dependent, as long as all the other criteria are met.
From IRS Pub 501
Example 2.
You provided $3,000 toward your 10-year-old foster child's support for the year. The state government provided $4,000, which is considered support provided by the state, not by the child. See Support provided by the state (welfare, food benefits, housing, etc.), later. Your foster child didn't provide more than half of her own support for the year.