To qualify as a dependent child for tax purposes, a child must meet specific criteria set by the IRS. The child must be under 19 years old at the end of the tax year, or under 24 if they are a full-time student. Additionally, the child must live with the taxpayer for more than half the year and cannot provide more than half of their own financial support. The relationship test is also important; the child must be the taxpayer's biological child, stepchild, foster child, or a descendant of any of these. Other requirements include the child not filing a joint tax return with a spouse, unless it's solely to claim a refund.