YF99,
You did not mention if your child is a US born citizen, or a naturalized US citizen. There is a difference in this case.
TheRealCanadian is right. A Birth Certificate of a US or Canadian born citizen is sufficient. Also acceptable is the Canadian Citizenship card, if you are a naturalized Canadian citizen. Since no such "citizenship card" exists for naturalized US citizens the only other option for naturalized US citizens is a US passport.
But a passport is always a better option. They will potentially hassle you less on the border. They have gotten more nervous since 9/11 (specifically the US border guards). A passport can easily be scanned in their POE system, while a birth certificate, and citizenship card cannot.