Both hikes are not easy. The Bradford Ridge hike is definitely a lot harder when you get to the descent into the canyon. Unless the entire group is in very good shape, I would not do it. Kids that age are usually of varying capacities and physical conditions. You are asking about the difference in difficulty: the Bowen trail is quite a bit shorter (around 2 miles versus around 3 miles each way), and the descent on the way in and ascent on the way out is much less extreme. But still taxing and difficult unless you are in very good condition. Is there a reason you want to take 20 young girls to the naturist hot springs? There are many hikes in the Los Angeles National Forest that might be less extreme, and more appropriate for the age group. Also hiking the PCT from the Lake Arrowhead area (Splinters Cabin, with access to many swimming holes) might be a better bet than the hot springs for this age group.