Owned by two brothers from Newton, CrossFit RedZone doesn't resemble a typical gym. Instead of treadmills and bench presses, monkey bars stand alongside hanging rings and medicine balls piled like an arsenal of kickballs. Such equipment might seem spare, but that's exactly what CrossFit is about. In each class, coaches challenge students' bodies in a rotating array of workouts, incorporating everything from kettle bells and crates to obstacles in the parking lot. As adults learn to redefine their physical limitations, kids' classes take into account children's unique physical and mental development to create workouts that prepare them for a lifetime of healthy habits such as bench-pressing broccoli stalks.