How do I be assertive?

2 days ago, I started working as a summer day camp councillor. I have a passive demeanour already, because of childhood bullying. I’m the new guy, so they put me with another councillor’s group. So me and the other dude (we are both 16) are in charge of 9 little boys who are 7 yrs old. Even since the beginning, the kids never listened to me and as soon as the other councillor turns his back or leaves me as the only one in charge, that’s when they break all the rules. They treat me like a pushover.