Although I think the idea of a book on Gay and Lesbian Heroes has potential, I believe that in order for it to be successful you have to take many things into consideration, beyond excellent writing that's acceptable to the majority. The why is simple -- it's a red hot button subject and it appears that it will remain so for a while. That said, if I were you I'd be working on it right now and changing it as needed according to voices heard and laws passed on a local, state, and federal basis, as well as voices heard from parents, churches, schools, and a slew of other who are in this dogfight. You have to become an "expert" layman in the subject area. And you'll definitely need several prestigious people and organizations to endorse your work. I went down this road many years ago with a book I wrote to be used by parents, teachers, and other caregivers to teach basic sex education for children (4 to 8 year olds). This at a time when the needfor same was glaring, but those in need weren't ready for it. Three years after its completion the atmosphere was ripe, even though I had to fight all kinds of dissenters (The John Birch Society & a slew of other well known organizations and individuals). Ultimately, Time Life Books bought the book and it went on to become an international best seller.
So while what you write and how you write it is crucial, so is research, endorsements from well know, credentialed organizations, associations, and individuals, a fantastic query and proposal --- and choosing the right time to try and get the work published. My belief is that you're going to need all of this if you want to give yourself the best shot to be published.
While I don't think you have to write the work in a dark, sullen manner, I do think you have to be unique and not display any bias in any way, shape, or form. Ths probably sounds easy. Believe me, it's not. That unique twist is the key. Staying above the maddening crowds is another. Being able to take the wrath of dissenters is but another of many. I don't believe you can think of a book on gays and lesbians, especially for children, as you would any other book on a simpler subject. But, that's just my opinion.
You say it's for MG, do you mean upper middle grade and/or young adult? I don't know how you could effectively pull this off with early middle graders.
Have you run the concept by any people who are objective about the subject matter? Parents, teachers, librarians, book store owners, oranizations and/or associations etc. I would. Lots of them.
I'm not crazy about the purple book because I think the market you describe is very limited and difficult to reach.
Good luck