can you put a govenor on a 4 wheeler so grandkids can drive safely?

What you talking about?

Don't know how you'd do it on a FI model. Carbed one, all you need is something to crimp on the throttle cable where the cable itself goes into the sleeve, you want to limit the travel.

Depending on the model if the throttle itself has a stop, then just loosen/lengthn the throttle cable adjustment at the carb so that at full push of the throttle, it only open the blade as much as you want it.

Charlie

I put a bolt behind the throttle of a go cart until my nephews were old enough to speed it up. I'm sore the same could be done for behind the thumb throttle of an ATV.

good idea - will check that

Yes, simply add a bolt to the throttle thumb as stated or if it is a carb'd you can simply place a sleeve inside the throttle assembly where it limits how far the throttle will allow it to come up...

I would also suggest adding a kill switch to it if they are beginner riders this way you can tie a rope to the kil switch and walk behind them as they ride and if they get to far or go towards something scary you can pull the kill switch off on them. it is attached to them as well....

If it's a slide carb, take the top off, pull the slide off the cable and put a piece of tubing on the cable and put the slide back on.
You will have to experiment on it to get the right length.
That's how we used to do it to kids dirt bikes until they got the hang of it.

After they get used to it, you can take the tube out and nothing is harmed or hacked.