Christopher Robin (or some other character) is a better protagonist, because Winnie the Pooh wouldn't be looking for himself, whereas Christopher Robin often came to the 100 acre woods looking for Pooh. Besides, making Christopher Robin the protagonist gives the person you're asking a sporting chance of guessing the answer -- not fair otherwise.
BTW: To understand the joke, a kid would have to be familiar with Winnie the Pooh and also be old enough to understand why this riddle is naughty. I'd say 5-8 yo is the prime audience.