SuperHeroStuff.com has some fantastically authentic super hero clothing, accessories, toys and products, and they have a sale section with a few good buys for little ones!
These Batman and Superman Kids Costume shirts are on sale for $18 each, which is 37% off the list price of $27. I think they could definitely be used as a Halloween costume, but you could get a lot of mileage as just everyday t-shirts after the holiday!
If parents are into superhero costumes, there are more than a few options for them, although they're not on sale at the time. The Caped shirts are available for men and women (Batman and Robin, anyone?) and this Bazinga! shirt cracks me up!
Finally, you can save 15% on DC Product orders (Batman, Superman, Wonder Woman and more) of $50 or more when you use SuperHeroStuff.com promo DCU15 at checkout.
You can save 11% on any order of DC products when you use promo code DCU11 at checkout.
Standard shipping starts at about $5.