I think it is beneficial to give kids a framework of some basic physiology so they can understand what you are saying, and so they have some familiarity with the terms they're likely to encounter while reading about training elsewhere. You don't need to drown them in technical terminology, especially if YOU don't understand it—a lot of hacks use big words to sound smart...don't be one of those guys, kids see right through that.
My kids, or at least the ones who listen, have a rough idea what VO2 max, lactate threshold, and aerobic threshold mean. When I say "lactate" they know what I'm talking about. I don't bother to go into the biochemical reasons why I don't say "lactic acid" because it's just not important for them. I'll lose too many in the details.
Eventually you want your kids to leave with an understanding of training so that they can continue to train effectively on their own, if they would like to. I think this is only possible if you understand WHY you do the workouts they do, and what effect they have on your body. A basic physiological framework also helps you figure out whether a certain workout or training program is bogus or not.
If you're looking for a place to start, just explain the basics of what "aerobic" and "anaerobic" mean when you talk about them.