Yeah, it gets confusing...really, get a passport for the baby.
And take the birth certificate as well.
What if.....you drive down, no passport, and you have to fly home quickly??
Nope, baby can't fly.
Just makes life oh-so-much easier
HTH!! from Gov Can site..
http://www.voyage.gc.ca/countries_pays/report_rapport-eng.asp?id=308000
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Air travel
Canadian citizens travelling by air to, through, or from the United States must present one of the following valid documents:
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A passport; or
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A NEXUS card when used at kiosks in participating airports.
This requirement applies to all Canadian citizens, regardless of age, including children.
Canadian citizens intending to fly to, through, or from the United States should apply for a passport, keep it up to date, and carry it with them when travelling.
The NEXUS program offers a simplified and expedited border clearance process to low-risk, pre-approved travellers.
Land and water travel
Canadian citizens must present one of the following valid Western Hemisphere Travel Initiative (WHTI)-compliant documents when entering the United States by land or water:
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a passport; or
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a NEXUS card; or
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a Free and Secure Trade (FAST) card; or
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an enhanced driver's licence (EDL) or enhanced identification card (EIC) from a province or territory where a U.S.-approved EDL/EIC program has been implemented; or
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a Secure Certificate of Indian Status.
Canadian citizens 15 years of age and under require proof of citizenship, such as an original or a copy of a birth certificate, or an original Canadian citizenship card. Canadian citizens 18 years of age and under who are travelling with a school or other organized group, under adult supervision with parental/guardian consent, may also present proof of citizenship alone.
Canadians who are permanent residents of the United States must comply with special entry requirements. For more information, see the U.S. Customs and Border Protection (CBP) Web site.
The most important formality on entering the United States is providing proof of your Canadian citizenship. Your Canadian passport is the best document to prove your Canadian citizenship and your right to return to Canada.