A quick google search yielded me these results:
CarGo Beep Beep!
Lettuce in, its cold outside!
Anita tissue....ah-choo! Too late!
Canoe come out to play?
Thats about it for the good ones IMHO. Also, I found this site call 101kidz.com, which seems to be decent. They had a kid particular section for jokes which yielded a few decent ones.
Q. What did the porcupine say to the cactus?
A. "Is that you mommy?"
Q. What do you get when you cross a snowman with a vampire?
Q. Where do snowmen keep their money?
A. In snow banks.
Q. Why do sea-gulls fly over the sea?
A. Because if they flew over the bay they would be bagels!
Hope some of those are useful. Good luck in the joke fest!