"A qualifying child for purposes of the child tax credit is a child who:
Is your son, daughter, stepchild, foster child, brother, sister, stepbrother, stepsister, or a descendant of any of them (for example, your grandchild, niece, or nephew),
Was under age 17 at the end of 2010,
Did not provide over half of his or her own support for 2010,
Lived with you for more than half of 2010
Is claimed as a dependent on your return, and
Was a U.S. citizen, a U.S. national, or a U.S. resident alien.
For each qualifying child, you must either check the box on Form 1040 or Form 1040A, line 6c, column (4); or Form 1040NR, line 7c, column (4)."
If your child tax credit is limited because your tax was less than the full child tax credit, you may be able to claim a refundable additional child tax credit. You can use Form 8812 to figure this credit.