''OK, Cast 2 Soldiers, take your rifles. Cast 2 Bunnies? Cast 2 Mousy?'' Ron Cunningham doesn't have to raise his voice. He just makes this request and a squadron of 10-year-olds shoulder plywood arms as a mouse and bunny take up their positions.

As the garbled boom of an old tape of ''The Nutcracker'' fills the Boston Ballet's rehearsal rooms, the soldiers of Cast 2 charge in formation against a bunch of men who represent the huge, evil mice attacking the Nutcracker, their leader.

''The Nutcracker'' is a magic world for the 200 children who appear in two casts of this annual production. For some, however, it's more than an incredible dose of make-believe. According to Nicole Eldridge, who is dancing the coveted role of Clara this year for the second time, this is the only ballet children can relate to ''until they're older or get more serious.''

Nicole, a pleasant, unaffected 11-year-old with Huckleberry Finn-like freckles, curly dark brown hair, and a turned-up nose, remembers getting serious at age 4 - when she first saw a little girl dancing Clara. When she dances in front of the mirror, she watches herself in a way that looks vaguely perky - until you realize she is smiling to mask her intense concern with just where this turn is going and when that foot goes down.

She should be concentrating. In this child-dominated ballet (there are 81 children's roles, 28 for adults) Clara is the focus. There are, of course, the Sugar Plum Fairy and the Dew Drop Fairy, danced by those lofty and distant creatures, the professional ballerinas. But it is before Clara's wide-with-wonder eyes that the whole two-hour fantasy unrolls. The 200 Boston School of Ballet children and 34 company members are scheduled, rehearsed, and deployed like an invasion through 32 performances in three weeks. There are two casts. There are understudies. And there are small heroes who take up the trumpet of an absent bunny and go on with no rehearsal at all. In these glitzy, gleaming, whirling, violin-wooed and fake-snowflake-dusted three weeks of December the Boston Ballet (like most major US ballet companies) earns most of its operating budget for the year. These children help, as do their peers on the other side of the footlights.

This tremendous effort doesn't necessarily lead to a career in ballet, says Ron Cunningham, but it helps them learn discipline for whatever they do. There is, however, a certain ballerina mentality already at work among the children. The role of Clara, for example, is so coveted that the girls who get it don't have to learn it: They have gazed so longingly at the little girls who were Clara before them that the steps have already sunk in.

In fact, the two Claras and their understudies, who were jostling around an empty rehearsal room one day recently, weren't even practicing their steps. Instead, they were sliding in and out of one of the divertissements the professional dancers do with a studied nonchalance. They put in long hours at rehearsals, lying on their stomachs among the ballet bags doing their homework when they're not needed. Then they hop up at a moment's notice to curtsy to the Sugar Plum Fairy or to wave while being towed offstage in an imaginary sled.

''I can picture everything,'' says Nicole. So when director Ron Cunningham (who also dances Drosselmeyer the Magician) points to James Reardon in an old T-shirt and tights, she walks around him with wide eyes and clasped hands - nodding her approval exaggeratedly because she sees the toy Nutcracker, magically grown to life size.

This vision is probably what got her the part - more than her erect carriage and her ability (noted in the Boston Globe) to kick her leg up as high as her ear. ''I cast Clara as a little girl, not as a ballet dancer,'' says Bruce Wells , acting artistic director of the company. What he looks for is ''presence - a sense of performance. I love Nicole because she's so unaffected.

''It's not a big reach,'' he adds, for the children in the audience to imagine themselves in her tiny pink shoes.

''Nobody wants to see a 31/2-foot ballerina,'' Cunningham agrees. ''They need to be natural and spontaneous, but they need training. A kid off a street wouldn't project that.'' He says Nicole is ''very talented - whether that means she'll be a wonderful dancer or maybe a very fine and wonderful lawyer,'' it's too soon to tell, he and Mr. Wells both feel.

Nicole, however, decided to become a professional dancer seven years ago when she saw her first ''Nutcracker.'' This is her fourth year in ''The Nutcracker,'' and though she will have a hard time giving up the role of Clara, she knows already what roles she should be working through to progress. She'd like to be a Flower, she says, adding that ''some people get Dragonflies, but not many.'' She studies ballet for an hour and a half to two hours a day, aside from Clara rehearsals. And she's apparently immune to stage fright. She's only sorry she can't see more than the first few rows of the audience, because when they smile at her, she works harder.

What you really need, she says, is ''to have the determination to work hard. Unless you're really, really determined, there's no way you can be a dancer.'' At four she told her parents ''I kind of wanted to be a dancer. . . . They told me how hard it would be and that they (dancers) don't get paid well or anything.'' That clinched it for her: ''I wanted to do something that I could work hard to get.''

In the midst of the set - a glittering blue Victorian living room with towering French windows, lavishly spangled Christmas tree, heaps of presents, and taffeta-bedecked parents - she is a focus. Her eyes are bright and seem to be seeing everything for the first time.

When she dances with the toy Nutcracker, holding it up and gazing dreamily at it as that foot flies up past her ear, the audience can't help looking at it, too. And when the Nutcracker turns into a life-size soldier, she stares at him with almost fearsome joy. Her delight when the room whirls and the tree shoots up to two-story height is wild. She rushes to and from the growing tree, her back to us, and she's so enveloped in this whole transformation - her favorite thing about being Clara - that it really is like a dream.

It is this conviction, ''picturing everything,'' that comes across as unaffected, charming, and childlike to the audience. Nicole really loves Clara's adventure. But whether they cast a ballet dancer or a little girl, what they got was a very determined person. She may look like she has sugarplums dancing in her head, but what she's really doing is working out a career.

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