Another thing to remember, IMHO, is to ask the question when dealing with teaching martial arts to kids, is "how far are you willing to go with the depth of training concerning the variables of age, emotions, and mental acuity?"
When training for physical fitness and SD, what level of SD are you going to take it?
What is not enough, just enough, or too much?
For example, are you wanting to teach them to surprise and deter the antagonist long enough to get away and seek help?
Or in Rob's example of older teens with an overzealous "boyfriend" that is going too far, the 'deter and seek help' method may just piss the guy off and escalate the situation...in which case have you taught to incapacitate until the law arrives?
I know it all depends on the age of the child, but what I'm getting at is the awareness level and the mental ability required for the child to know how to judge the required amount of force necessary to do the job that needs to be done.
Here's where the quality of the teacher would be paramount.
I hope that made sense.