But to return I have seen one site say just the birth certificate while another says a passport card, similar to our enhanced ID's. Anyone with kids travel frequently back and forth by car that can straighten this out? Thanks!
The rules are different depending on whether you travel by air or by land/sea. If that sites you are looking at make different assumptions about how you're crossing the border, that could explain the apparent contradictions.
Our kids are often asked to identify themselves. As US residents, we tend to get more questions entering Canada than returning to the US, I presume because that's where they're looking to prevent a child being taken out of his country improperly. The most questioning came the time my husband was driving the kids up without me (I had flown ahead). The agent said to our then-9yo, in the fake chirpy voice adults often use with kids, "Are you happy to be going to Canada?" The answer was a grim, "NO." The agent visibly tensed and asked, as neutrally as she could manage, "Why not?" "Because," said the 9yo, "our great-grandmother is DEAD and we are going to her FUNERAL."