My wife, daughter and I (all US citizens) are planning to fly to Toronto and back.
"Entry into Canada: Canadian law requires that all persons entering Canada carry both proof of citizenship and proof of identity. A valid U.S. passport, passport card, or NEXUS card satisfies these requirements for U.S. citizens. Children under 16 need only present proof of U.S. citizenship."
However, to get back into the US, according to https://help.cbp.gov/app/answers/detail/a_id/3618/kw/travel:
"Children are also required to present their own passport when traveling by air."
"U.S. citizen children ages 15 and under arriving by land or sea from a contiguous territory (Canada or Mexico) may present an original or copy of his or her birth certificate"
In other words, it seems as though a birth certificate is sufficient to travel from the US to Canada via air. However, in order to return by air, a passport is required (the birth certificate is only valid if traveling by land and sea).
HOWEVER, I see things all over that say a birth certificate is acceptable for both border crossings. For example, on https://traveltips.usatoday.com/things-required-back-canada-109007.html:
"If you're traveling to and from Canada with children under the age of 16, you must provide an original or a copy of each child's birth certificate. Other acceptable documents for American minor children include a Consular Report of Birth Abroad and a naturalization certificate. Children don't need a U.S. passport for this trip."
Needless to say I'm really confused. I don't see anything that says my daughter won't be allowed into Canada, but how about coming back?