My kids keep asking me theological questions I can't answer. Yesterday my five year old insisted he wanted to know "What does G-d look like?" I had no idea what to say....
Sometimes the best thing you can tell your child is "I don't know." You teach your much child more by being open about your inability to answer a question, than if you would give a half-baked answer just to get off the hook.
If you don't know something, but fudge an answer, you teach a child that it is more important to look like you know something than to be honest and look ignorant. That's a bad message. Saying I don't know teaches that it is alright not to know everything, and it's ok to be honest about it.
Also, by saying you don't know, it shows your child that when you do have an answer, that answer is a real one. Your answers have more credibility when you only say what you really know.
But even more importantly, by saying, "Great question, I don't know the answer, let me try and find out," you teach your child that learning never stops, and everyone can learn more, even a parent. This is the greatest lesson you can teach your child. You may not have given him an answer, but you will have inspired him to ask more questions.
PS. My answer to your child's question, "What does G-d look like?": Great question, I don't know.