Can a guardian claim the child tax credit for their ward?

If they are eligible foster child-

eligible foster child
(1) Child defined (A) In general The term “child” means an individual who is— (i) a son, daughter, stepson, or stepdaughter of the taxpayer, or (ii) an eligible foster child of the taxpayer. (B) Adopted child In determining whether any of the relationships specified in subparagraph (A)(i) or paragraph (4) exists, a legally adopted individual of the taxpayer, or an individual who is lawfully placed with the taxpayer for legal adoption by the taxpayer, shall be treated as a child of such individual by blood. (C) Eligible foster child For purposes of subparagraph (A)(ii), the term “eligible foster child” means an individual who is placed with the taxpayer by an authorized placement agency or by judgment, decree, or other order of any court of competent jurisdiction.