It seems very appropriate and wonderful that you are thinking about talking to your SD about this kind of thing. Certainly you'll want to coordinate with the bio mother and your partner, but I think adults talking to their kids about sexuality is very important.
A very good book for kids right around 9-11 years of age (it may or may not be too early for your SD, depending on maturity level) is "It's Perfectly Normal" (by Robbie Harris maybe?). I'll admit to being shocked at all that is shared in that book at first (including some drawings), but it is shared in an age-appropriate way, and I think after my shock I realized what value it had.
Another thing to consider would be taking a mother-daughter sexuality class for girls around that age. This might help open up the lines of dialogue. Often it is easier for kids to hear from their parents about bodies and sex when it is in the context of a class, with other supportive adults who help to normalize it. Often hospitals with family education programs have such classes. Your local Planned Parenthood also may offer such classes, if you are open to that. Depending on your willingness to attend a class from a religious organization, a Unitarian Universalist church in your area may offer Our Whole Lives (OWL) classes for kids just slightly older than your daughter (there is a 4-6th grade curriculum), so it'd be a short wait. The classes are just for the kids, but parents are given a complete orientation, the kids are assigned homework to do with their parents that encourages dialogue, and the parents usually read a "Parent Guide to OWL" that has great, very specific non-judgemental and non-"religious" tips and examples about talking about these subjects.
|I'm not sure what to say about the kissing. It depends on why she is doing it (because she feels affectionate toward them? because someone is pressuring her? to embarrass the kiss-ee?) and what standards you want to teach her about when kissing is appropriate. Sounds like you need to hear more about what she is feeling to understand what's going on here.
Aside from telling you that I do think it is relatively normal for kids to go through kissy stages right around your SD's age (I remember kissing boys on the playground at about 6, going through a break, and then getting back into that kind of stuff when I was about 8 or so...and that was many years ago), I want to say that I agree with EnviroBecca.