It happens every year: Father Time creeps offstage with his scythe, while a diaper-clad New Year strides triumphantly into the spotlight. Thanks to Jack Nicholson and Rebecca Broussard, Bruce Springsteen and Patti Scialfa, Steven Spielberg and Kate Capshaw, among others, 1990 brought a rich crop of new kids to the block. Spielberg's latest project, Sasha, is even more adorable than E.T., the Boss has somebody who's clearly born to run, and Nicholson is doing the paternity shuffle for the second time his firstborn, Jennifer, is 27 (and was an assistant production designer on The Two Jakes). On his performances as a proud papa, the old joker says, ''(It's) not my most flamboyant role, but I've always liked it.'' Long life and good health to all of the above, and happy 1991 what better way to ring in the New Year than with a rattle?