The whole point of kids' hybrid bikes is that children can ride them off-road, as well as on roads, sidewalks, or paved cycle paths. They're great for riding over grass, on dirt tracks, through forests, and so on, but they're not designed for hardcore off-roading over extremely rough, rocky terrain. For this kind of riding, only a mountain bike will do. That said, most kids are only likely to engage in light to moderate off-road cycling, such as family bike rides on unpaved trails or rides around the park, and a hybrid bike is just fine for this.