Her Canadian one had expired just 3 days prior, like OP, and the GB was not expired (dual citizenship, Canada born). When we asked while leaving Canada at the YYC Airport, was totally acceptable without anything else required. Coming back was the problem and we ofcourse take partial blame didn't do enough research and probably should have asked (perhaps a post here on Beyond to inquire migth have saved us the trouble haha).
In the end, we needed an eTA (Electronic Travel Authorization), here's a CBC article from 2017 on it (which was the year we had this happen to us):
https://www.cbc.ca/news/politics/ele...iner-1.3921453
Not sure if this eTA thing could help with your 16 year old's situation, might be worth a quick phone call to inquire though!!