Kids as I’m sure your aware are extremely smart. Personally I would be more worried about them turning over the engine with the engine in gear and running over one of their siblings. Not sure what I would recommend, but I’ll share what I’ve seen done.
Quick story about myself and my dad kid (and hired hand proofing the M). I don’t remember this but apparently I was 2 or 3 and down in the barn with my dad. He had the M hooked to the manure spreader inside the side shed of the barn where the barn cleaner exited. Apparently I was unsupervised for a moment and managed to both pull the ignition button and hit starter button. The tractor always started real well and of course was in gear (reverse). I’ve was told it ended up putting a hole through the side shed of the barn and I was headed toward the silo before somebody got me stopped. My dads solution was to remove the starter cable from the switch and everytime he went to start it he would touch the bare cable off the batter post. Not the way I would recommend but that kept me or anybody else from ever using that machine.