An FSA may be used to pay for care for self, spouse, and dependents, including a child who would be a dependent except for divorce or separation.
However, I do want to note that both parents can't legally file for HOH unless one child actually physically lives more than half the year with each parent. The IRS follows physical custody--where the child actually lived--and not state court custody orders or decrees. It is certainly possible to arrange custody so that one child spends more that half the year with one parent, and the other child spends more than half the year with the other parent, but you have to physically arrange that. It only has to be one day (183 nights with one parent and 182 nights with the other parent), but you have to make it happen. (And of course, if the parents only separated in 2024, it might be the case the children spent more than half the year with both parents because of when the split occurred. But for 2025, you may have to work to get that "more than half the year" requirement for each child.)
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