When I bought my son's 50cc it came with one on it. Didn't work to well. On my daughter's quad 50cc also, after her first runaway she learnt very quickly to let go of the throttle. Now they both want more throttle. They learn quick and with some guidance they will be alright. Also you can limit the throttle. So the short of it, tape a block to the throttle, put on a helmet and chest protectors on and teach your little ones the right way:d
You can buy the remote kits at Princess Auto that include a remote and a switch controller last time I checked a few months ago,.. but they might also have been surplus.
Of course you would have to wire everything up into the ignition circuit.
Start jogging. My hubby did that for all of 10 minutes when 5 year old was on Honda 50cc bike. Kid caught on real fast. Actually for the first 2 or 3 short rides hubby sat on back, in the yard. Then he started jogging alongside.
You can also buy the kits on Ebay but the remote kits usually come with an alarm system as well. I had it on my sons 90cc & it would kill the battery if not run for a while. I removed it & educated him on throttle control.
Our sons 90cc has the shut off but actually other that goofing around with the "big boys" we never used it. Just educated, helmet, chest protector and like the one comment already said only took one little accident and our boy learned the about the brake fast!!!...