http://www.amazon.com/Its-Not-Stork-Families-Friends/dp/0763633313/ref=pd_sim_b7 This is part of a longer series of books. It is recommended for ages 4-8 - big span and of course it will depend on the family. This is not a book for people who prefer to avoid giving their kids information, but if you are comfortable with biological information we found this series to be a good one.
Opinions will vary but I come down strongly on the side of providing information. Sex is an important part of life. Being evasive can give kids the message that sex is something that is shameful and that you aren't a reliable source of information. Too many kids rely on peers (or the Internet) for information and when that happens you've lost the opportunity to be a source of influence. If you want your fourteen year old to talk to you - answer your four year old's questions.
