US to Canada by air - minors passport expired

From the CBSA web site:

Identification requirements for U. S. citizens and permanent residents

If you are a U.S. citizen, ensure you carry proof of citizenship such as a passport, birth certificate, a certificate of citizenship or naturalization, or a Certificate of Indian Status along with photo identification. If you are a U.S. permanent resident, ensure you carry proof of your status such as a U.S. Permanent Resident Card.

For all modes of entry, we recommend you carry a valid passport for all travel abroad, including visits to Canada from the United States. A passport may be required by your airline or alternative transportation authority, as it is the only universally-accepted identification document.

So for US citizens, proof of citizenship (a US birth certificate) is sufficient to enter Canada. You will want to check with your airline too, to see whether they have additional identification requirements such as a passport.

Also from that page,

Border services officers watch for missing children, and may ask detailed questions about any minors travelling with you.

It doesn't sound like your situation will appear suspicious, but nevertheless it is wise to be prepared for further questions.

For returning home to the US however, you may encounter possible difficulties. From the CBP web site:

Air Travel: All U.S. citizens departing from or entering the United States from within the Western Hemisphere by air are required to present a valid passport or NEXUS card (if utilizing a NEXUS kiosk when departing from a designated Canadian airport). Merchant Mariner Document (for U.S. citizens on official maritime business.) Note that children are also required to present their own passport when traveling by air.

There is no exception for children listed there (but there is an exception for children 15 and under arriving by land or sea, not air). A recently expired passport and a US birth certificate may be acceptable, but I wouldn't count on it.