If I take my kids to Canada without my wife, do I need a release documents from her?

Yes. When traveling across the border, with minor children, you require a letter signed by any parent not present or a court order authorizing the trip. Border services also recommends that the letters be notarized. It should the full names of the child, who they are traveling with, that parents contact information (preferably a cell phone number), the date/time of the crossing, where the children are going/staying, and the expected return date.

Child custody disputes, child abduction, and trafficking/prostitution of children is a serious concern for border officials and they don't make exceptions.

If you are flying via air, you need passports.

If you are married, then I don't see why. I would take the childrens birth certificates along with their state IDs and yours as well. Can never be too safe in other countries! You can contact your local DMV and see if they have any suggestions.