Since the courts issued an order of guardianship for the child then in IRS terms they are your child ... read the rules in PUB 17 starting on page 29 ... https://www.irs.gov/pub/irs-pdf/p17.pdf
Foster child. A foster child is an individual who
is placed with you by an authorized placement
agency or by judgment, decree, or other order
of any court of competent jurisdiction.
