Since it's summer and you can't make practices mandatory, there's really nothing you can do about the kids "skipping" the optional practices. I run my summer practices not as cheer practices but as fitness and conditioning practices. I do a CrossFit approach and post a "WOD" (workout of the day) on our team website (everything is done using body weight-air squats, ab work, lunges, running/sprinting, push up variations) and the kids can either do it on their own, or show up to our practice and do it with me and whoever else shows up. They are required to email me by a certain time to let me know that they will be coming (I don't want to bother driving over if no one can make it). This way the kids who work during our evening work outs, or who are away on vacation or at a camp etc can still do the daily workout.
I tell the kids that it is not mandatory, and that there are no consequences for them not showing up, but if they choose to spend the summer on the couch watching MTV and eating crap and not doing the workouts, they will have a consequence when the season really starts, they won't be able to keep up with the team and won't have the same strength as the rest of the team. If that means I pull them from a stunt group, so be it-that's called natural consequences. NFL training camps are starting up and I sent out a team email with a link to an article about NFL players who showed up to training camp and failed their fitness tests and had to miss out on "real" practice and instead do things like ride stationary bikes, run, lift.....not what they were there to do, and it was letting their team down.