Family themes flourish in Broadway musicals this season. Varied forms of this often missing element can be seen in current fare

Broadway musicals and family themes are rather like mixed metaphors. They don't normally go together. This season is somewhat different. Of 12 book musicals lighting up midtown stages last week, five were family-oriented. The transfer of ''The Human Comedy'' from the downtown Public to the Royale Theatre (where it closed after a brief run) and the reopening of ''The Tap Dance Kid'' at the Minskoff have accentuated the phenomenon.

''The Human Comedy,'' which Galt MacDermot and William Dumaresq adapted from William Saroyan's warmly touching and comic World War II novel, casts its own special glow over the Broadway scene. A sacrifice of intimacy was inevitable in moving the pop opera from the 275-seat, arena-style Public Anspacher Theater to the 1,058-seat Royale. Director Wilford Leach made changes to suit the larger space and the Royale's proscenium stage.

Where possible, Mr. Leach used the aisles for exits and entrances. The innovation allowed for some spectacular dashes by young Homer Macauley (Stephen Geoffreys), the mercurial Postal Telegraph messenger of the tale. An upper box served as the caboose from which the Trainman (David Lawrence Johnson) sang his ''Hi Ya, Kid'' to little Ulysses Macauley (Josh Blake).

But it is the performance that must draw the spectator into this lyrical version of Saroyan's world. With the original New York Shakespeare Festival cast almost intact, and with conductor Tania Leon's sensitive baton work, ''The Human Comedy'' once more worked its magic.

On yet another family front, ''The Tap Dance Kid'' takes an unusual course for a black musical. Whereas extravaganzas like the long-running ''Dreamgirls'' steep themselves in show business, ''The Tap Dance Kid'' concerns a conflict over whether the son of a middle-class black family belongs in the business at all. The musical was adapted by Charles Blackwell (book), Henry Krieger (music), and Robert Lorick (lyrics), from Louise Fitzhugh's novel, ''Nobody's Family Is Going to Change.''

On his mother's side, young Willie comes from a family of tap-dancers. Willie dreams of following in their footsteps. The lad's father, an upwardly mobile lawyer, opposes the idea and forbids Willie to continue studying with his tap-dancing uncle.

''We didn't get off the plantation until we stopped dancing and started doing ,'' William (the father) angrily tells his son.

Haunted by inherited memories of slavery, William sings:

Dancin' like they did on the plantation,

Do you think I don't know how?

The old moves they ain't forgotten,

It comes back like pickin' cotton. . . .

Being a well-tempered Broadway musical, ''The Tap Dance Kid'' resolves its conflicts. How could William fail to relent after the show's series of breathtaking tap numbers? Hinton Battle sets the whirlwind pace, with young Alfonso Ribeiro following his lead, while Alan Weeks and Hattie Winston contribute their own fancy footwork. By evening's end, the family has got it together and the tap-dance kid is on his happy way.

The first musical of the 1983-84 season was also its first hit - and the first to touch on family matters. But with a decided difference. ''La Cage aux Folles'' (Palace) was taken from a French play that inspired two film comedies. It deals with the dilemma of a young man who must acquaint his fiancee's conservative parents with the fact that his family consists of a homosexual father and the father's longtime partner - who is also the star drag queen of their St. Tropez nightclub. The lavish spectacle has so dazzled Broadway theatergoers with its razzle-dazzle and Jerry Herman score - and with the strong performances by George Hearn and Gene Barry - that audiences have taken its bizarre situation in stride.

In a more circumspect gesture to family life, ''Baby'' (Ethel Barrymore) focuses on three couples for whom having a baby creates enough comic dilemmas, sentimental touches, and convenient music cues for a small, cheerful musical. Performed by a personable company, ''Baby'' was written by Sybille Pearson (book), David Shire (music), and Richard Maltby Jr. (lyrics and direction). Being in a family is a way of life for ''Baby.''

Of all the season's domestically oriented musicals, ''The Rink'' (Martin Beck) covers the widest time span and the most emotional ground. It begins in the present as wreckers prepare to demolish the once-proud roller-skating rink of the title. Terrence McNally's book moves backward and forward across the years to tell an often downbeat mother-daughter story while reflecting on contemporary phenomena ranging from marital infidelity to neighborhood decay. Not your everyday Broadway material.

''The Rink'' is blessed with two charismatic stars in Chita Rivera and Liza Minnelli. Their superb performance of a fetching Kander-Ebb score is epitomized by ''The Apple Doesn't Fall.'' It delights the audience and closes the generation gap.

Five shows on family themes don't mark a trend. But they do make an impression, even if their concurrence is merely coincidence. With so much attention being focused on the parlous state of the family, it seemed worth noting that certain fashioners of Broadway's most typical fare have found something in the old institution to consider - and even to celebrate.