We read an awesome book over the summer to do a little introduction into plant genetics--- it talks about Mendel and his pea plant experiments and ties it a little to family traits.
It's a wonderful book, well written for 5 grade, would help introduce a lot of vocabulary on the subject, and has helpful images: http://www.amazon.com/Gregor-Mendel-.../dp/0810954753
What parts of reproduction are you covering? I did this on a high school level so we were pretty blunt with what goes on, but I know this will be different for 5th graders. Our guidance counselor is actually teaching them health which makes me happy since I'd probably freak out even having to teach kids about stuff like that.