I am the legal guardian of a child, do they qualify as my child?

Yes, that is considered a foster child for being a Qualifying Child. 

To qualify as a dependent, a child must also pass these tests:

• Relationship: Be your son, daughter, stepchild, eligible foster child, brother, sister, half-sister or -brother, stepbrother, stepsister, adopted child or the child of one of these
• Age: Be under age 19 or under 24 if a full-time student, or any age if permanently and totally disabled
• Residency: Live with you for more than half the year, with some exceptions
• Support: Get more than half their financial support from you
• Joint return: Not file as married filing jointly unless only to claim a refund of taxes paid or withheld

Congressional Research 2-10-2022

“Most types of relatives of a child meet the relationship test: parents, grandparents, aunts/uncles, and siblings all meet this test. Legal guardianship—where the child is placed with an adult by a court order— would meet this test (the child would be considered a “foster child” under this test), regardless of the actual biological relationship between the child and taxpayer.”