Who is a qualifying child?
A qualifying child is a child who meets the following tests:
Relation - The child must be your son, daughter, adopted child, stepchild, foster child, brother, sister, half brother, half sister, step brother, step sister, or a descendant of any of them (such as a grandchild, niece, or nephew)
Age - The child must be under age 19 (under 24 if a student) and younger that you (or your spouse) or permanently and totally disabled
Residency - The child must have lived with you over half the year (exceptions for divorced or separated parents, see IRS Publication 501)
Support - The child must not have provided more than half his or her own support
Joint Return - The child must not be filing a joint return (unless the return is only to claim a refund of withheld income tax or estimated tax paid)
See IRS Publication 501
for more information, exceptions and examples.