Others have answered, but I will add as well.
The throttle limiters are also widely available on the motorycles (actually the 4 wheeler stuff migrated over from the bikes)
But, as others have said, the 4 wheeler builds more confidence in the stability then what is actually there. The flip happens VERY quickly.
Please don't take me wrong, I think teaching your kids / grandkids to operate machines is very important. I think you should do it. My 2 cents is too get them whatever you ride, and they will follow along with you, then you are participating with them.
I don't know if credentials matter too you, but I have fair credentials when it comes to motorcyle and 3 and 4 wheeler safety and a number of years of experience, and that all really boils down to you have to do what you feel comfortable with for your kids and grandkids.
If you get a machine without a throttle limiter, contact me and I can help you add one easily at no cost.
Also, be aware that throttle limiters on two strokes with a centrifigul clutch have some inherent problems, in that to get it to rev fast enough to engage the clutch, if they are in an open field, can build a suprising amount of speed as they hold the throttle open. I tend to practice throttle control, and played red light green light with my kids (and others) for hours in the yard.
I attached a picture of my son, it is one of my favorite pictures of all time with him. He was about 8 at the time of that shot.