Sounds like he fell off and was run over.
Yup, those things move at 1-3mph. It would bump you and you’d either get run over or step out of the way once it bumps you. Sounds like he fell off and got under the tire.
There was a guy evacuating his family from a hurricane in FL and he ran over his 3 year old son in the yard and killed him.
I’ve got lots of power equipment, power tools etc around here and I back up a truck and trailer in the yard quite often and have two kids so I’ve had some scary thoughts and even visuals to the point where I had to put it out of my mind because it was so disturbing. I can only imagine going through some tragedy with one of my kids. Not sure how or if I could handle it.