Great question — answering as the UphillTech team:
Canada requires passports to be valid for the duration of a visitor's stay. There is no requirement for additional validity . You could bring a notarized letter from the children's parents permitting you to travel with them. It's unlikely to be necessary, but if anyone does decide that something untoward is going on, the letter should help dispel their suspicions.
Full answer by phoog: https://travel.stackexchange.com/a/143305 (CC BY-SA 4.0).
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