Things must be a lot different in the States. It wouldn't cross my mind to be concerned if we went into Quebec with extra kids or if our kids were with others. The only thing I'd want to make sure they had was their provincial health card. Otherwise …
If we were crossing into the U.S. that would be a completely different thing. It came up with rep hockey and passports (obviously for all) and notarized letters are required for kids / teens travelling without their own parents.
Are there issues crossing into different states?