Edith, my seven year old asked me why God created mosquitoes. (We really need to take her religious education in hand; I think right now most of it is coming through her West African nanny.)
Since I don't know why God created mosquitoes, but do have the answer to how mosquitoes evolved to fill an ecological niche -- which is much simpler to explain -- I went with that. I threw in Darwin's visit to the Galapagos Island's and his finches.
I guess the best answer to the intelligent design folks that I have heard lately is that the design is hardly intelligent. If it were, then we wouldn't have bad lower backs, too many wisdom teeth for the space in our jaws and appendixes. Not to mention the Platypus (unless it shows that God has a sense of humor).
What we really need is a foundation that would pay for putting a copy of the Origin of Species into every hotel room along with the Bible (and in the case of Marriott, the Book of Mormon).