It is straight forward from the view of the IRS. Only the custodial parent (the one with whom the child lived with the most nights during the tax year) gets to claim the child care credit. It doesn’t matter who actually pays it. Even if the custodial parent allows the other parent (by giving them a signed form 8332) to claim the dependency, the custodial parent still claims that credit.
When you write 50/50 custodial schedule, since there’s an odd number of days in the year, one of you had the child for the most nights. If you can agree which parent had the most nights that’s fine. If you can’t agree and can’t document who had the child the most nights the IRS would give the dependency to the parent with the higher AGI.
