I agree with you, the difference is what are perceivable dangers and what not. The problem about the treadmill is that for a child (and an adult) the treadmill

I agree with you, the difference is what are perceivable dangers and what not. The problem about the treadmill is that for a child (and an adult) the treadmill is not immediately dangerous, in particular the danger of being sucked under. Modern playground design is actually taking this into consideration by designing with manageable dangers (climbing up high ladders etc.) but removing things that are not obviously dangerous, like for example gaps where heads can get stuck