For some reason, this is a classic for ALL ages. We play it in camp with team kids, and they get more creative with climbing and hiding. With the younger ones, they love having an obstacle to hide in or under.
I use panel mats and parachutes as little forts. Those big donut mats work great as a fast hiding spots for your group. These pictures are actually of a big easel that we were using. Those girls would go in and out and in and out for a long long time,
Hiding not only creates fun for the class and athletes, but you can create fun hiding spots that work on skills. I use things like sit in the fort and count your toes in a pike, do 10 v-ups, hold a plank, try a split, etc. Kids find it fun, and you can work in the basic drills and conditioning that can be boring.
One last tidbit, as a coach, one of the best ways to get a hold of you class again is to grab all your kiddos attention by saying "come on! let's go sneak over hear and hide!" They immediately jump in every single time. It's a super easy way to get those distracted kids back engaged in class.