Child must be in your custody - per IRS. Special rule for children of divorced or separated parents.
Even if you can't claim your child as a dependent,
he or she is treated as your qualifying person if:
• The child was under age 13 or wasn't physically or
mentally able to care for himself or herself, and
• You were the child's custodial parent. The custodial
parent is the parent with whom the child lived for the
greater number of nights in 2018. If the child was with
each parent for an equal number of nights, the custodial
parent is the parent with the higher adjusted gross
income. For details and an exception for a parent who
works at night, see Pub. 501.
The noncustodial parent can't treat the child as a
qualifying person even if that parent is entitled to claim the
child as a dependent under the special rules for a child of
divorced or separated parents.
