It’s always tricky in an exercise class that is open to everybody who wants to walk in. I use those balls as well for many clients but for me it’s one person at a time, and I know their abilities.
It may be a good idea to have some non-weighted balls for those new to the drill.
Ultimately, it will be up to you to decide what you want to do in your class. It is a compromise between challenge to some and safety to all. Unfortunately, not all participants know to assess their own abilities and will try to participate immediately at an intensity beyond their capabilities.