The staff at Performance Bike in Rockville, MD are fabulous.
Any question you have about cycling in general, for kids or adults - well, they have the answers!
When the fit you on a bike, they actually let you ride it (with a borrowed helmet and some ID, of course) out on the sidewalk for a bit. No other bike shop I've encountered will let you do that!
They only carry one brand of bike, Giant. They are a bit pricy, but they do tend to have end of season sales.
They do carry a full line of bike accessories, from packs, to panniers, to trailers, to camel-back hydration systems, to clothing and helmets. They also have a full line of bike parts for total customization.
They also have a bike repair shop, and can build your bike for you - though in the spring it can be a few day wait to actually get your bike after purchase.
All in all - a great resource for bike info, accessories and high-quality bikes.