School children want me to be mean to them?

I work at a school part-time. My job requires me to be part babysitter, part teacher.
My problem is that sometimes they simply do not listen to me. Their teachers ask them to do something simple (like line up) and they do it no problem. When they are asked by me, its like pulling teeth.
After a few months of this, I asked them why they wouldn’t listen. their reply was, “You’re nice to us, and other people are mean”
I never in a million years expected that answer. They act out because I am too nice? I thought I was being pretty fair, not letting them get away with much, but not punishing them for every little small offense.
I dont want to be the mean lady, but I dont want to continue to be disrespected by kids either. I thought I had found a balance but apparently not.
Any suggestions?