There's a MG book by Patricia MacLachlan called Unclaimed Treasures where a girl of about 12 has romance at the front-and-center of her mind, so it's a big part of the "plot" as it were. In fact, she has a crush on the man next door, but ends up realizing his son (who is her age) is a much more reasonable first love. There is kissing and discussion/thinking about kissing (she practices on her bedpost).
I always think P. MacLachlan's writing is charming, but I admit--I liked this book more when I was younger. I recently re-read it after about . . . twenty years, I guess. This time around, I guess my reading was colored by the fact that I now have a 12 year old of my own. I found it a bit squicky. I'm just not the biggest fan of MG kids kissing, etc., and being obsessed with falling in love (and I know that's just my personal bias as a late bloomer).