Manslaughter is the crime of killing a person without meaning to do so (an accident). It is not considered as bad as murder, which involves the intention to kill. For example, if two people have an argument and start hitting each other because they got angry, and one of them kills the other, that would usually be called manslaughter. Killing someone in self-defence is not always thought to be manslaughter.
Children are manslaughtered every year. Children are unrightfully killed at such a young age.