TOM PERROTTA's thirtyish parents of young children are a varied and surprising bunch.  There's Todd, the handsome stay-at-home dad, dubbed "The Prom King" by the moms at the playground, and his wife, Kathy, a documentary filmmaker envious of the connection Todd has forged with their toddler son.  And there's Sarah, a lapsed feminist surprised to find she's become a typical wife in a traditional marriage, and her husband, Richard, who is becoming more and more involved with an internet fantasy life than with his own wife and child.