I look at it in terms of "spontaneity" - what happens to the object when nothing intervenes and everything goes as normal? (I've heard it also described as "perfective end" although that may not be referring to something slightly different).

A sperm is not a child because it doesn't spontaneously become a child (i.e. left in a system without intervention, the sperm will not spontaneously become a child). A fertilized egg, however, would be considered a child because in a system without interference, it will grow and grow, eventually exiting the womb. Hence why the pro-life group (myself among them) considers abortion to be murder, and why disingenuous arguments like "masturbation is also murder, so in contraception, so are wet dreams" look ridiculous by comparison.