IN THE BEGINNING hardly anybody thought Bostonians in sufficient number would wander over to the old Faneuil Hall marketplace to buy chocolate chip cookies. And only a visionary few could imagine Baltimoreans flocking down to the waterfront to line up for french fries. But Quincy Market and Harborplace are now jam-packed realities, and their architect, Ben Thompson, has done it for Washington.

Still, the cynics are asking: Will Washingtonians venture downtown, particularly in the evening, for an Indonesian egg roll?

In part that will depend on the quality of the egg roll, but we won't know that until The Pavilion at the Old Post Office has had time to iron out its kinks. We have hopes. The restaurants look promising: Blossom's grew out of Lily's in Boston's Quincy Market. Fitch, Fox & Brown is an offspring of Bootsie, Winky & Miss Maud of Dupont Circle; Richard's has branched out from Pier 20 with its seafood menu; and Cafe Maxime, now expanding to five locations around Washington, is an old friend from Mazza Gallerie. And there is still an opening in The Pavilion for another restaurateur adventurous enough to pioneer the downtown revival.

A mainstay of the "restored downtown marketplace" concept is an array of specialized eating bars. The Pavilion's are Indian, Indonesian-Chinese, Middle Eastern; pizza and tacos and french fries; hot dogs, barbecue, deli foods and Buffalo chicken wings. Not as vast as Harborplace, but a welcome change from the Mall hot dogs for tourists and from the downtown theater district's dearth.

Not only does "Dinner with Julia" premiere on television Oct. 13 (8:30 p.m., Channel 26), but dinner live with Julia will be a major Washington event Oct. 2. For the 22nd anniversary party of WETA, Child will join all those willing to pay $75 a ticket for a dinner featuring her recipes catered by Ridgewell's, an ice sculpturing demonstration, and informal talk between Child and Jean-Pierre Goyenvalle of Le Lion d'Or, who was guest chef on the final show of Child's series. For tickets to the dinner, at Mazza Gallerie, call 998-2695.