> Danger is important for children.

This is very, very true. I didn't use safety gates on my carpeted stairs when my kids were babies, and they learned to navigate stairs almost as soon as they could walk. I let them use scissors when they were really little too, and by the age of three they were expert little paper crafters.

That said, I sure as shit wouldn't let them play around a running treadmill, even if I was in the room.