@childofthecross I work in a daycare and I've seen this sort of behaviour before. I've had perfectly fine children, who can be sassy at times and also know inappropriate things from their peers but also know to never say these things out loud. Good kids all around.
Cue the troubled child joining the daycare who has a lazy father that enjoys letting his kid watch inappropriate shows and doesn't monitor his internet usage. He comes into daycare swearing, pushing kids, yelling and talking about sex, drugs, violence and porn. These topics of course rouse the interest of the well-behaved children and they'll find themselves attracted to this other kid. They'll pick up his mannerisms and their behaviour starts to shift, becoming more physical with each other over disagreements, teaching even younger children this "forbidden knowledge" and finding it cool to swear when they're mad or upset.
It's a corrupting effect that is potent because of their age. Case and point, that boy leaves for two weeks and like day and night, these kids stop acting out as much and by the last day of this kids vacation, they're acting well-behaved as they once were.