Suggestions Needed: Humorous Children's Fictions.

Also, check out:
The Peter and the Starcatchers series by Dave Barry and Ridley Pearson...they are a great take on the Peter Pan characters.
The Great Brain Series by John D Fitzgerald
Here Comes Charlie Moon by Shirley Hughes
13 Ways to Sink A Sub by Jamie Gilson
The Famous Stanley Kidnapping Case or The Headless Cupid, both by Zilpha Keatley Snyder
James Howe books...he might be a little old for these but I still find them hilarious
My Sister the Vampire...Nancy Garden
The My Teacher Is an Alien series by Bruce Coville

books by australian author Paul Jennings.

If you can do fantasy, Vivian Vande Velde has some books that are kind of funny.... at least whimsical.