Jen from The Crafting Fiend made the cutest pair of ruffle bum jeans for her baby daughter, due to arrive any day now. They have ruffles around the hem, and rows and rows of ruffles across the seat. She shares a the pattern and a tutorial showing how she made them. The pattern is for a baby 0-3 months. If you needed to make a larger size, you could draft your own using her pattern as a shaping guide and a pair of pants that fit your child as a sizing guide. Go to her blog for the pattern and tute.
[photo from The Crafting Fiend]