If you came to an agreement on who will claim that child on their return then you will claim that child only. However, if you're undecided then no one can claim the children as the Canada Revenue Agency will verify that the children are not claimed twice.
According to Canada Revenue Agency (CRA), if you and another person each have a clearly established requirement under a court order or written agreement to make child support payments for a child, normally neither of you would be able to claim the amount for an eligible dependant for that child. However, in this case, you may still be eligible to claim the amount for an eligible dependant, as long as you and the other person agree that you will make the claim. If you cannot agree, neither of you can claim an amount for an eligible dependent for that child.
If you make child support payments for a child and the other parent does not, you cannot claim an amount for an eligible dependant for that child. Only the parent who does not pay child support can claim the amount for an eligible dependant on line 30400 of their tax return (Step 5 – Federal tax) for that child.
For more information, you can visit the Canada Revenue Agency (CRA) website - Child Custody and the amount for an eligible dependant
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