Not entirely accurate or up-to-date, but it is a good beginning for kids. Besides, it provides the opening for a conversation on how science is fallible and constantly changing to correct itself.
Why does the puffer fish puff?
Why does the giraffe have a long neck?
Why is the cactus so popular?
Why is the platypus so unusual?
Why are tortoises different?
Why is Lonesome George lonely?