Agree with everyone else so far. YOUR wedding, YOUR decision as to who attends what. If your friend doesn't like it, too bad - and seriously, what's all this nonsense about her DD being disappointed that she's not going to be flowergirl? If you'd asked her to be flowergirl and then taken the invitation back, sure, I might be a bit offended, but if you never offered to begin with (or, I assume, it was a tentative 'depending on x, y and z', and it ends up being that she can't be FG), that's HER problem to deal with for putting ideas in her daughter's head to begin with!

Don't let it get to you, honey. If you back down for their sake and hold your wedding ceremony somewhere you don't absolutely love, you'll regret it. And I'm sure they will regret their pettiness if they refuse to attend your reception for the sake of making a point. Do things your way, hun - it's your special day, and we all do it differently. Whatever works for you