If you and another person, in this case, a current or former spouse or common-law partner, share custody of a child throughout the year and you each have a clearly established requirement under a court order or written agreement to make child support payments then neither of you can claim the amount for an eligible dependant. But, in this case, one of you may be able to claim the amount, as long as you and the other person agree on who will make the claim. If you cannot agree, neither of you can claim the amount for an eligible dependant.
For more information on claiming an amount for an eligible dependant, please use the CRA link below:
https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/support-payments/shared-custody-amount-eligible-dependant.html