Our recommendation is to start small. We mean it. 2-3 kids for a month. Run the class exactly as we lay it out. When the kids are comfortable with the format and you are comfortable as a trainer add a couple more kids. As the class gels open it up. It may take 4-6 weeks to reach this point, but it will save you in the long run.
Now as the question:
Preschool 1 trainer for every 7 kids
Kids 1 trainer for every 10-12 kids
Teens 1 trainer for every 12-15 kids
These are starting numbers. BBoJ has had 35 kids on her own. I have had 30+ teens on my own. But we have been doing this for several years.