X is not, despite what most alphabet books say, for xylophone or X-ray. At least, in neither of those words does it stand for its usual sound value, which is the two-phoneme k-s of box, six, and sexy. The best way to introduce this sound to children is through the familiar XXX they write at the end of letters. Try saying "kiss" without the i and you've got an x sound.
The z-like initial x of xylophone and Xerox occurs occasionally in Greek source words. Otherwise x is found inside or at the ends of words. In a few - such as luxury and obnoxious - it makes a kzh sound, and sometimes in the prefix ex it sounds like gs (exact and exist), but usually it is a well-behaved ks. And as Scrabble players know, it doesn't crop up that often. It's as a symbol rather than a sound that X comes into its own. As well as signifying a kiss, it marks the spot on treasure maps, shows you've got a sum wrong, and stands in for an illiterate's signature. It represents Christ in Xmas, and any incognito Mr, Miss, Ms or Mrs in divorce cases. It is the great unknown in algebra, and a signifier of specific chromosomes, rays and particles in other branches of science. And in the plural, it represents the strength of beer. All of this is of no comfort to phonics experts writing alphabet books, who end up having to change the traditional "X is for . . ." to "X is in fox, fax or taxi." It feels so untidy.