With kids, give Beacon Hill / Acorn Street a miss.
The most photogenic walk in the city — gas lamps, window boxes, and Acorn Street's cobbles are pure New England postcard. The catch: it's a residential neighborhood, so there's nothing to 'do' but wander and shoot, Acorn Street is tiny and often crowded with photographers, and it's better as a 30-minute detour than a destination.