The angry woman in me thinks: I will teach mine to violently put down any aggression towards them. Get punched in the face for now reason? Keep hitting them until a teacher pulls you off. Nobody will mess with you because it'll just be too risky!
But the rational woman in me thinks: enrol them in Judo.
I've done martial arts my whole life and i can tell you that sending your kids to Karate or Tae Kwon Do will be fairly useless for self defence; not to mention the "only horses kick!" response teachers tend to give to a child using Tae Kwon Do's trademark kicks. Schools tend to give a generally negative response to any striking style, i've even heard phrases like "I know he was punched first, but your child has had training in deadly techniques!".
The judo club i go to has a kids class, and they have so much fun. There's less focus on complicated throws and things, and more just general rolling around and learning to control another person's body. Regarding self defence, some of these kids who've been training a little while are brilliant against new kids who show up even when those kids are older/larger. They can gently trip them to the ground and hold them down... not hitting/choking/etc them, just pinning them down so that they feel smothered and uncomfortable. They also have a great base and are quite difficult for untrained kids to push/trip over.
In a school setting, i think a kid would get into a lot less trouble if the teacher found them merely holding the other person down after being hit themselves; a great alternative to the shocked response schools would have if your child's footprint was found on the other childs jumper after kicking them, or the dreaded bloody nose from a punch! A kid doing some basic judo just looks like a kid grappling aimlessly with another, but a kid doing Karate looks like Bruce Lee. Also, since there's not prancing about doing pointless 'kata' and kicking into the air as there is in Karate training, and instead there is live sparring against a resisting opponent, Judo is thus very effective. Good for fitness too.