I am planning on telling my kids to jump rope for 15 minutes before they can take a break to rest.
Are you actually going to force them to exercise that much?
It's true. The kids are having fun jumping.
That's good that they're having fun and for them to get exercise. You need to let them decide when to stop, though. They know their own limits and it can be dangerous to make them overexert themselves.
They won't die from it. They are not physically active enough.
Well just make sure that they are in shape enough to jump rope for that long. It's important for kids to get regular exercise, but make sure to supervise them. Especially if they are not normally physically active, you need to look for any signs of them being overly exhausted.