Can a child defend himself against an adult? Probably not, how is it possible for a 5-year-old to resist someone who weighs many times his weight and is incomparably stronger and taller. A 5-year-old with a height of about one meter can apply self-defense techniques to someone who weighs 80 kg and is 1.70 m.? Probably not.Can a child between the ages of 5 and 12 manage techniques that can cause permanent harm to a peer? Probably not.
At this age, real self-defense is the knowledge that parents need to pass on to the child. About how he will behave when he is at school among other children, what to do and what not to do when he is out with friends, when he comes in contact with strangers, if he gets lost, etc.
This is a real approach to what we need to do to make our children as safe as possible when they are not under our direct supervision.
Sending the child to some self-defense classes and not teaching him the above does not add to his safety, it just creates a false sense of security that can have painful consequences.
The child should know what he (she) is able to deal with , what he should avoid (which is the most preferable), how he will avoid it and what he should do next.
We will come back to self-defense now from an age of over 12 years old children can start taking self-defense classes. This helps to increase their self-confidence, to be able to cope with many problems and at the same time strengthen their body. This always in combination with what you have taken care to teach them before is going to protect them from any danger. In short, it is better to be able to "see" in advance any difficult situations and to avoid them than to be in need to deal with them.
As for self-defense, the only system that has an answer for all scenarios is Krav Maga, which is why it is used by the members of the special services around the world.