You can easily find costume for little baby, even newborn and older, school kids too. Costume for newborns and babies up to 6 or 9 month can be blue romper with S sign and little red cape. You can put your baby on blue jumpsuit too but make sure it has leg snaps for easier diaper changing.
The same goes for toddlers too. They'll look adorable in little Superman costume.
Image by ksyam, Public Domain License, Clker
Most costumes are comfortable, made of quality materials and quite affordable.
Girls can wear Superman costume too - they can be Supergirl, the girl version of Superman (she was his cousin) or Lois Lane. Supergirl's costume is in the same colors as Superman's but girls get to wear short blue skirt and red boots (in costume sets these are usually boot covers). Another cheap, fast and simple option is T-shirt. There are T-shirts with red cap sewed on or just shirts.
And of course you can make Halloween costume at home too. You'll need blue shirt and tights and little red skirt for girls. You can get Superman logo at Clker (public domain) and put it on the shirt or buy shirt with S sign.
Red cape can be red piece of fabric or your old red T-shirt.
As I already mentioned there are really a lot of costumes available online and if you decided to buy costume online, make sure you order it soon enough in case you'll have to return it (it may not be good enough or too small or too large) and buy new one.
And it's always good to know what other buyers have to say - what's why it's always good to read few customers reviews and see if they were pleased with it and what to expect before you buy Halloween costume.