Personally, I'd let them have the books and not worry about it. If they have questions and you're open to answering them, they'll ask. If they don't ask, it probably means they're ignoring that part of the book.
Remember that kids on the farm know how baby animals get made from a very young age. They have living examples of how babies are made that are out in the pastures making babies! Knowledge isn't going to do them any harm even from an early age. Both my kids knew the basics of how babies are made by 4.5, and they're not out corrupting the neighbor kids. LOL! To kids at that age, it's just more factual info. No big deal. At worst, they'll think it's gross.
If I may make a suggestion, I would recommend that you have a clear idea of where you stand on things like premarital sex and how you want to present your moral views from the start. You don't want to be fumbling through that conversation if a child decides to take it there. And if you start talking about your moral views from the start, the kids will tend to accept them more easily.
If your child is the sort to lecture anyone and everyone about his latest interest, and reproduction becomes his latest interest, you might want to talk about appropriate forums for discussions about sexuality. A kid old enough to talk about it is also old enough to understand that it's not a conversation that the old woman in line at the grocery store or the 3yo neighbor is going to need to have...
