I really do understand your concern. I haven’t spent that much time in Thailand, over the years it probably adds up to about 6 months. In this time I must have seen 10 motorbike accidents that have definitely killed people. Ask me the same about the UK where I have lived for 28 years and it’s 0.
My first thought was to do as the fire station chief did when my mum caught me messing about with a fire in the back garden; show them a load of grizzly videos of dead people to scare them off. Trouble is kids have a way of thinking ‘that won’t happen to me’.
I think in Thailand it’s unlikely you will stop them going on mopeds if they want to go on them