I'll recommend the books my mom used with us, mostly because she did it when we were really little. She wanted us to know stuff properly and not learn it on the playground.
My body is private - the title is pretty self explanatory, here's a link to it on amazon. http://www.amazon.com/Private-Albert-Whitman-Prairie-Book...
If you type in the title, it gives you lots of other ones to choose from on the same topic.
Before I was born. This book talks about body development for both boys and girls. It alludes to conception, but doesn't go into details, mostly to show that the egg is so tiny and look what it grows into... It has lots of pictures of children, adolescents and adults to show the body's development. Here's the link, though I didn't see the exact book we used this time, there are some similar ones. These have more of a "let's talk about sex" theme with a Christian emphasis. I don't know if that's what you're looking for, but there are also some about body development here as well. http://www.amazon.com/s/ref=nb_ss_gw?url=search-alias%3Da...
Once you've found a good way to have "the talk", be sure she understands that you're interested and any and all questions she has as time goes on. Its especially important for her to be aware of this and able to talk to you, rather then experiment with other kids.
Also predators often seek out mentally challenged kids because they think they can get away with stuff, so they are at a higher risk. I worked with a blind camp once, and most of those kids are also mentally challenged (that is often the cause of their blindness) and nearly everyone had been molested and people wouldn't believe them for a long time. I don't mean to scare you or dwell on the negative, but it helps to be proactive against this type of thing early on!
Best wishes as you tackle this subject with your daughter!