This is what my reference said in 2016.  You should just confirm that there has been no change since then, but I do not believe there has been:

"Where two children spend part of the year with each parent, or each of them spends the whole year with one of them, and no child support is paid, each parent may claim the eligible dependant amount for one of the children."

If there is only one child, they can alternate years or one can claim, but not both.