Interesting questions and applications. My kids are 8 & 10, and love to play swords, etc. I recently got a three-pack of Cold Steel trainers, and the boys each wanted one. I said sure, because they're no more dangerous than the plastic swords they already own. In playing with them, I expect they will naturally learn some technique from imitating me. But it's not instruction per se. Certainly not drills.
Another thing is that kids see a sharp difference between play fighting and real fighting; I remember once they were spouting off about what they would do if a bad man tried to kidnap them - it was all the stuff they are learning in Karate class, legitimate strikes, all right, but stuff that won't stop a real man in a real situation. I stopped them and pointed out that if a man really were to try and put them in a van or something, they should scream as loud & as long as they could while trying to gouge out his eyes and rake at his face, punching and kicking for all they're worth. I freaked them out, because I burst their self-confident bubble (which I hated to do), and also because suddenly they could imagine what I was talking about, I made it more real, and it scared them.
