Toontown in MK is a great place. We've seen Cinderella, Snow White, Belle and Sleeping Beauty there. There are different Princesses there each time though so I wouldn't mention who will be there to your kids until you know for sure whose there.
There's actually one line to see the Princesses but it can get very long so get there early when Toontown first opens around 9:45am usually. We've even seen the Queen of Hearts and Tweedledee and Twiddle Dum hanging out in the Hallway as you leave. They are a hoot.
Ariel has her own grotto in Fantasyland. This is another line that can get long.
In Epcot World Showcase, you can find Princesses. Jasmine is in Morroco along with Aladdin, Snow White has been seen there - I think in Germany, Belle is there usually in her every day dress but I can't remember which country. For some reason when we were there in June 2005 all the lines for the Princesses in World Showcase were really short. That was 3 years ago though so it may have changed.
You can also ask any CM and they can find out locations for you. Have fun and Good Luck on your Princess Hunt.