Sertis enters the lion's den.... And has a mysterious dream about his parents.
- 11 year old have almost no sexual interest ... they haven't matured enough. While your harry may have budded early from his Death Eater Activities it's unlikely that any girl in gryffindor will have. You didn't say how old or which year this girl was that came on to Harry but I am assuming that they were the same year as no self respecting teen is going to day dream about a prepubescent 11 year old... they'd be laughed out of the common room.
Don't push so hard... let things happen naturally in the story. Storys don't have to revolve around or have a ship to be successful.