Upvote:4
Unfortunately, since you are traveling by air, you do need to have a passport (or Nexus card) for the child or you won't be allowed to board a flight to the US, as @phoog says.
You didn't mention which airline you are using, but you can easily look up the travel documents required and confirm for yourself. For example the official Air Canada site says:
Travel Documents:
Canadian and U.S. citizens must present either a valid passport or a valid Nexus card when travelling by air between Canada and the United States.
Other airlines will be the same.
In fact the (unofficial) document you linked to also states:
If you are returning to the U.S. from Canada by air, you can only use a U.S. Passport as proof of citizenship.
There is a bit more leniency at land crossings and IIRC some exceptions for youth sports teams, but not for air travel. A casualty of 9/11.