So many parents are scared that their child will ask a tough question and they won't have the right answer. This is backwards. You should be much more scared that they will ask someone else the tough scary questions in life. If you want your children to follow in your faith, and maybe even be stronger in their faith than you, then you should encourage them to ask questions. Even if you have no information on a topic and you get caught off guard, you can still respond positively. Here are some sample ways to respond for a topic that you might not know much about.
**** Example Conversations *****
Parent: That is a good question! Thank you for asking me that question. This is a topic I would like to learn more about too. How about we buy a book / contact the youth pastor / research this together?
Child: Hey Dad/Mom I am not sure I believe in Truth/God/Jesus/Christianity/Going to Church/etc...?
Parent: That is great that you are asking yourself this question. Thank you for sharing your doubts with me. This means you are starting to develop your own beliefs instead of just borrowing them from me and mom. We should believe what is true, not what our parents say is true. How about we start seeking the truth together?