How to assist with 3-5 year old children?

So I am nearly 17. I am a dancer and the managers of the studio decided I could help the teacher Kayla (she is 21 and teaches two of my 4 classes) with intro children. It is 4 hours every saturday. I have the basics down- help correct positions and teach technique and stay on the opposite side of Kayla to help the kids not on her side, and help with bathroom breaks, etc. I don't really know how to talk with little kids though. Last week a little girl decided to sit down and didn't want to dance and Kayla asked me to try and help out with that situation, but I couldn't get through to her. Then, in the tumbling class, I have to make sure the kids who already had their turn sit and stay quiet and off the mat, but a 3 year old decided to run around and the other kids started following her. I got the 5 year olds and a couple 4 year olds to sit, but the three year old didn't listen and neither did her followers. I didn't chase them, of course, because my dog handling kicked in and said that if it's a bad idea to chase a running dog, it's a bad idea to chase a toddler. How can I start being able to talk to these children and getting them to trust and respect me? What else can I do to help make this year go smoothly? The classes range from 5 to 12 students, btw.