Preschool (daycare expenses) have to be claimed by the spouse with the lower net income, and one of the limitations is 2/3 of earned income. If your wife had no income in 2015, then there is no claim for preschool.
While there is no claim for preschool, it is considered educational, and therefore does qualify for the Arts credit. You are able to claim, as long as you have receipts in the children's names, up to $500 per child for the Arts credit.
