Killing a young boy- what type of phrase is this?

It means the act of willfully, by any means, ending the life of a male child. A child is defined by American culture as generally being below the age of 18; though if the child is described as young, this generally means 12 or so and below.

Killing is another Word for murdering, which is homicide. Homicide is against the law in almost all countries, and is punishable by death. It is decidedly worse to kill the very young, like children, as most people view this as especially heinous.

There is no difference in feeling depending on the sex of the child, as far as I know in most countries.