Things like https://en.wikipedia.org/wiki/Creole_language and children of immigrants often not learning their parents' language well make me skeptical of the importance you're assigning to parents as teachers.
There's many a motivation for a parent not teaching their native language to their children -- my step-father's parents didn't teach their kids Spanish because they didn't want them to have an accent (and also got beaten as children for speaking Spanish in school as that was the '30s rural New Mexico way).
