According to Tax Help for Form 2441:
"SPECIAL RULES FOR DIVORCED/SEPARATED PARENTS
If you were divorced, legally separated or lived apart from your spouse for the last 6 months of the year, then your child (even if the child is not your dependent) may be a qualified person, provided all of the following are true:
- You had custody of the child for a longer time during the year than the other parent.
- You or both you and the other parent provided over half of the child?s support during the year.
- You or both you and the other parent had custody of the child for more than half of the year.
- The child was under age 13 or was disabled and could not care for himself or herself.
- The other parent claims the child as a dependent because:
For more information refer to Form Instructions for Form 2441 and the Publication for Divorced or Separated Individuals."