I think this is a GREAT idea. I would even say starting at 4th grade (though this might not be your mission) is a great idea. Children are reaching puberty younger on average than we did a generation ago. But I remember even in 4th grade, reading those Judy Blume books about puberty and having so many questions. That year someone started wearing a bra, and there was the first "boyfriend/ girlfriend" in class. And I went to a parrochial school in a small town 30 years ago. When I taught Jr. High in the 90s, I heard my share of the kids bragging about who they had sex with. *shudders* and I can't imagine what it is like for kids that age right now.
I love that you will be doing mission work with them. That's fantastic to show young girls their power to improve their community, to serve, to create worth from their lives, to earn a "thank you" from a stranger! I think that's probably the best thing anyone can do for a young person, and by extention, their community. It also gives them a focus other than boys and bodies, which are taking up a lot of mental and emotional energy. I recently heard someone say this: "The problem with society is that people decide what they want to do before they decide who they want to be." You're giving them a glimpse at who they can be and are opening a whole world of positivity up for them.
I fully support you asking a nurse to come in and talk sex with them. In this generation, every sixth grader knows about oral sex, yet they are getting information from other kids, who know nothing. Getting straight, true answers about what is going on and what the real physiological consequences can be, and teaching them morality in the same forum is great! Also, you're creating a community of girls who are bonded in knowledge and faith. I think that is a good combination because they have each other to lean on when peer pressure gets tough.
I wish you all the best. Thank you for doing this!