If getting a bar isn't a big deal then that's your best bet. They should do conditioning and drills warm up while you change it out, which can be performed on the other bars such as casts, holds, etc. Then you do your swings portion in the middle of the time, and more drills at the end when you change it (such as straight arm backward/extension rolls). I don't know what level we're talking about so I'm just giving really general ideas, but it would be possible to accomplish a lot structured that way.
You can do straps but with 30 mins factor in the time it takes for each kid to strap in with your help...that's the problem, it's not always time efficient with novice/younger gymnasts.