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If your order stipulates he must be current with support in order to claim them and he is behind in child support then no you can counter claim that he was behind and thus you claimed them.
If he is current with support (This is not medical support) then he had a right to claim the child and if you failed to allow him to do so he has a claim. The judge may allow him to claim your son next year since he was prohibited from claiming your daughter this year.
I wish I had better news,
Yes......if he is permitted t claim her via the IRS standards and the court order grants him this right then he would be permitted to claim her regardless.