Sometimes, everyone needs a rest. Some kids won't need a rest, but if a kid needs a break, please let them. They'll be back up and ready to go in less than a minute. No worries needed :)
Now before you think about kids getting lazy in class, just know this tip is for your beginners and parent tot kiddos. I see parents who want to get all their kids energy out...but they don't seem to realize that kids tend to be marathon runners when it comes to class.
Rest time is a good singing/stretching time to reinforce learning during class. Then they'll be up a hundred miles an hour with better gymnastics afterwards.