The teen instructors at my daughter's gym use the following strategies with great success:
1. When the kids start to get out of hand, say "If you're listening, do ___" (e.g., put your hands on your head). The ones who are not listening quickly start wondering why everyone else's hands are on their heads and quiet down so they can figure out what's going on.
2. If you are working with the kids one at a time, make the ones who are waiting sit down instead of standing in line, or give them a simple activity to do in place (e.g., practice cartwheels between two hula hoops instead of down a line).
3. Split up friends and "talkers" into different groups.
4. Dole out LOTS of praise for focus, effort, and following instructions.
I don't think it's necessary to turn yourself into a circus clown to get the attention of 5 - 7 year-olds. Most kids this age love getting praise and attention from teens and young adults, and if you consistently praise positive behavior most of the kids will soon be falling all over themselves to please you. One of the assistant instructors in my daughter's class is about your age and very soft-spoken, but she is actually more successful than the high-energy adult coach in charge of the class at getting the little ones to pay attention. It appears to be because she is careful about limiting their opportunities to get distracted and start climbing on things, uses a very structured lesson plan, and praises the kids for listening and following directions.
You must also be very strict about safety rules. If a little kid climbs on equipment she's not supposed to be using, crosses the vault runway, or runs in front of/under another gymnast or group, she could hurt another gymnast along with herself. Unsafe behavior requires an immediate and specific reprimand in a serious tone of voice, and appropriate punishment such as a time-out or the loss of a turn. Parents will not be mad at you--they will appreciate it.