You do not need a passport for an 8 year old, just a birth certificate. My wife is from Canada and we cross the border often, when she goes alone with the kids (5&8) I just write a note that I am aware she is traveling to Canada, and the reason I am not with them. She has also done it several times without a note.
http://www.us-passport-service-guide.com/does-a-child-under-l6-need-a-passport-to-enter-canada-and-return-to-us.html
http://www.us-passport-service-guide.com/does-a-child-under-l6-need-a-passport-to-enter-canada-and-return-to-us.html