Based IRS' Qualifying Child test (see #3 below), your foster child is not a qualifying child and doesn't count as a dependent because the foster child didn't live with you for half of 2017 (Aug-Dec is 5 months).
According to the Internal Revenue Service Publication 501, Exemptions, Standard Deduction and Filing Information, foster parents may be eligible to claim a Dependent Exemption for each child in foster care they care for during the tax year who is eligible to be considered a Qualifying Child. IRS has certain tests to see if the foster child is a Qualifying Child.
What is a Qualifying Child? A qualifying child is a child living in your home who meets the 5 Internal Revenue Service (IRS) tests/rules to be considered a qualifying child. The tests are:
1. Relationship (the child must have been placed in your home by the state or an approved agency for you to foster)
2. Age (the child must be under age 19 by the end of the tax year, age 24 if a student or any age if permanently disabled)
3. Residency (the child must have lived with you for more than half of the year) Note: The time period must be during the specific tax year between January 1 and December 31.
4. Support (the child cannot have provided more than half of his or her own support for the year) Note: The foster care board rate you receive is considered support from the state - not from the child.
5. Joint Return (the child cannot
file a joint return)
If all five of these tests are met, the
child is considered to be a qualifying