Saying "I don't know" is never acceptable.
If a guest asks you a question, you always have to have an answer, no exceptions. If you don't know it, find out, but don't say you don't know. If it's a silly question, make up a silly answer. If a child asks you what Tinker Bell eats before her flight across the Magic Kingdom during the fireworks, you better come up with an answer quickly. (Answer: very tiny apples.)