But seriously, the sooner you teach them the safer they'll be.
It's age/maturity dependent, but I wouldn't traumatize them with a .44 mag or a .50 cal right off but start slow with a .22LR and work up from there. It's surprising how quickly they are ready for a little more than a .22 LR but remember to go as slow or fast as they want to. The idea is not to scare them away from guns but teach them gun safety and pass on a way of life (gun culture, freedom, liberty, marksmanship skill, survivor mindset etc etc) from generation to generation.