I Just Got Hired/ A Little Scared?

I'm 21, and just got hired as a noon hour supervisor watching elementary school kids at lunch/recess time. It's a 3 hour a day job, and the pay is nice. However, I'm very timid, and pive. I was bullied a lot in elementary school until even some of college, so i have self esteem problems. Whenever i watch my little cousin, who is 12, he never listens to me. I'm scared I'm going to be bad at this job...