Are you thinking of making the costumes yourself? If so, go by your local JoAnn's (or even Wal-Mart) and check out their pattern books, Some of them have special Disney patterns (in the halloween/costume) section, with things like Peter Pan, Woody, Buzz etc... They also have pirate costume patterns to (in kid and adult sizes).
Are you and your DH going to dress up to? I think it makes for really great pictures if everyone is in similar costumes (i.e. all pirates, or from the same movie).
You can also buy costumes at WDW when you get there, especially around MNSSHP, they will have lots of costumes available (not as many for little boys as girls, but still a decent amount).
Also when it gets closer to Halloween, disneyshopping.com will have costumes for all ages (they have a few now, I think in the sale section).
Some things I've seen during MNSHHP -
1.) Toy Story: Kids as Buzz & Woody, (maybe Bullseye for the youngest?) Parents as Bo Peep or Jessie and Zurg
2.) The Incredibles (sure you'd have 2 dash's instead of a violet, but that's not a big deal)
3.) Pirates (of course)
4.) The Fab 5: Kids as Donald, Goofy and Pluto, Mom and Dad as Mickey & Minnie
5.) 100 Acre Wood Gang: Pooh, Piglet, Eeyore, Tigger, Honey Pot (Disneyshopping.com usuall sells these costumes around Halloween...but be warned their fur and can be very warm)
6.) 101 Dalmations: All Kids as dalmations, mom as Cruella (or entire famile as Dalmations)
Of course their no rule that you have match...you could just let each pick their favorite character. ...Or you don't have to go as a disney character at all!
You can also just google "Disney Costumes" and you'll come up with many many sites selling them year round, if you don't want to make them.
Hope this helps!!