Publication 596, Page 11, Rule 9:
Sometimes a child meets the tests to be a qualifying child of more than one person. However, only one of these persons can actually treat the child as a qualifying child. Only that person can use the child as a qualifying child to take all of the following tax benefits (provided the person is eligible for each benefit).
The exemption for the child.
The child tax credit.
Head of household filing status.
The credit for child and dependent care expenses.
The exclusion for dependent care benefits.
The EIC.
The other person cannot take any of these benefits based on this qualifying child. In other words, you and the other person cannot agree to divide these tax benefits between you. The other person cannot take any of these tax benefits unless he or she has a different qualifying child.
