Here's one of the things that made the biggest difference in our kids program: we stopped trying to teach everyone the same way.
Different ages aren't just different sizes, they're different minds. A six-year-old needs movement, play, and repetition. A ten-year-old needs challenge, autonomy, and context. When you design a class that tries to serve both at the same time, you end up serving neither as well as they deserve.
Separating by age group and building curriculum specific to each stage isn't just better for the kids. It makes your job as a coach dramatically easier, because you're finally teaching to the room you actually have.