''Kid'' is the operative word in describing the Young Guns approach to the Billy the Kid legend, since these films regard Billy as more of a gun-toting cut-up than a true desperado. As played again by Emilio Estevez in Young Guns II, Billy tends to giggle at tense moments and, during important ones, to mumble.

The other members of the Lincoln County Regulators, Billy's gang, share a similar boyishness that's geared far more carefully to the tastes of today's teen-age audiences than to the folklore of the old West. Indeed, whenever the film risks any hint of authenticity, the director Geoff Murphy is apt to try backlighting, slow motion, hand-held-camera work or some other pointedly anachronistic technique.

So Young Guns II, which tells of the split between Billy and Pat Garrett (William Petersen) that eventually led to the whole gang's apparent annihilation, concentrates principally on the drawing power of the post-adolescent heartthrobs in its cast. This approach has its appeal in limited doses, but it makes for a western that's smaller than life. (Advertising art for the film, incidentally, takes the bold move of positioning the story's six scowling junior outlaws against the backdrop of Monument Valley, with the actors in various sizes apparently determined by their relative stardom. This has the effect not only of reminding an audience how unimposing the actual characters are but of suggesting that the agents' battle over star size must have been a lot more interesting than what wound up on the screen.) Mr. Estevez, who first appears in old-coot makeup during a prologue that suggests Billy actually got away, is a sturdy if unspectacular presence, best when he's not asked to take center stage. He is given able support by Kiefer Sutherland, as the schoolteacher who's the most level-headed member of the group, and Lou Diamond Phillips, as the flinty half-breed who fields ethnic insults from the loudmouthed Arkansas Dave Rudabaugh, played by Christian Slater. Mr. Slater has done himself a huge favor by abandoning the Jack Nicholson mimicry that was his early trademark, and developing a very strong stellar presence of his own.

The screenplay, by John Fusco, works hard at colorful invective (''a bucket of mule dung like you'') and a lot less strenuously at keeping the action afloat. Billy's trademark cries of ''Yoo-hoo, I'll make you famous!'' at anyone he plans to shoot only heighten the material's silly streak. So do the story's occasional bathroom jokes and the practically teen-age madam (Jenny Wright) who in one scene rides off a la Lady Godiva.

A larger problem with the screenplay is that it's never quite sure where its sympathies lie. Billy and his gang are presented affectionately enough for a while, but their eventual sad fate is depicted without emotion. The fact that these outlaws became famous, and are being impersonated by attractive young stars, seems to be deemed reason enough for the film's existence. This image-conscious Billy is seen on several occasions savoring the newspaper clippings that made his reputation.