Here are a few great places to look:
This is a thread where people post pictures and list things that they make for their trips:
http://www.disboards.com/showthread.php?t=2179996
Here is a thread for a "Mickey's Grand Welcome" if you want to decorate their room from Mickey, Tink or the Princesses:
http://www.disboards.com/showthread.php?t=2342599&highlight=footprint
Look in Mel's thread for candy wrappers/pockets:
http://www.disboards.com/showthread.php?t=1980430&page=36
Also, look at Telynn's
http://www.disboards.com/showthread.php?t=1787936&highlight=nugget
There are lots of fun ways to announce the surprise. I like wrapping a big box and hiding helium filled mylar balloons inside with an note attached to the string of one of the balloons. Then your surprise announcement is just like the Disney commercial. Make sure you do this inside so that your balloons don't float away!
You could also go buy yarn in different colors...one for each child and string them all over your back yard with the surprise note at the end or different words about the surprise along the way, like "We" "Are" "Going", etc.
Some people love to wake them up and when they get to the airport hand them a present with a t-shirt inside saying "surprise we are going to Disney".
You could also decorate each child's room with fun banners, balloons and presents of things for Disney either as they sleep...so they wake up to all the excitement or while they are at school or out of the house.
That should get you started! Hope your trip is extra magical!