I do a little demonstration to scare the kids into being cautious around equipment, I always tell them that there are dangers even when the machine is not running and though I always lower all implements when parking the machine, there's always a chance that I may forget especially if just shutting down temporarily to work on something off the tractor..
So I take a plastic milk jug or some old plastic or metal toy and put it under the raised concrete filled ballast box, with the engine off and all quiet, when I have there full undivided attention I'll just casually but quickly move the lever full forward and boom that heavy box comes down hard and totally squashes what ever was under it, the harder plastic or light metal items make some impressive noises when getting crushed.
You can see by the look on their faces that they get it, that a peaceful, fun, big toy looking thing like a parked tractor can do them harm, I don't allow children to play on my tractors and I've yet to catch them doing it ether. I do this demo when ever a new play mate is over also.
Not trying to cause them any permanent fear as I do allow them to operate the tractors sometimes, under my close supervision.
There's also the problem of starting the machine when the 3 pt is down but the lever is in the raised position, that can cause a whole slew of other problems, always make sure the lever is in the lowered position, I've made this mistake a few times.
JB.