You may claim an exemption for your qualifying children provided you (and your spouse if you file jointly) cannot be claimed as a dependent by someone else and the citizenship or resident (21.8) and joint return (21.9) tests applicable to all dependents also are met.
Qualifying children include your children, siblings, and their descendants (see the relationship test below) if a residence test and age or student test are also met. If the relationship, residence, and age/student tests are met, you do not have to show that you provided more than half of the child’s support, as is required for a qualifying relative (21.4). However, a child is not a qualifying child if he or she provides over half of his or her own support. A married child who files a joint return also cannot be your qualifying child, unless the joint return is filed solely to obtain a refund. For a qualifying child, there is no gross income test; he or she may earn any amount and still be claimed as your dependent. Even if a child is not a qualifying child, as where the age/student test or place of abode test is not met, you may still be able to claim an exemption for a child with little or no income as a “qualifying relative” (21.4).