Family friends or strangers...

Maybe the people that see Peter's tiny little baby penis isn't a family member or somebody of his school. Maybe is just a family friend, or some stranger that caught Peter nude, or in a situation where his tiny weiner is exposed...

Between the family Friends that could caught Peter, we could mention Christine, Peter's mom coworker, Serena, Ginger and Stacy, a friend of Emma from her previous high school...

In the case of the strangers, Peter could be caught naked by a visitor of the waterpark, or from Big Burger, where Peter and his Friends usually eat...but it could also everywhere and by anyone...