D.B.
I would have stopped this about 3 emails ago.
The woman wants a destination wedding, and her husband of 1 year is coming up with all kinds of ideas about how to give her what he thinks she should have. He's trying to include his best friend (your husband) as the provider of a "temp" flower girl when the regular "employee" is out. The men are doing all this stuff that the woman doesn't want. She's seeing you as the pushy one, and you keep feeding into that by assuming that she engineered this request, which I'm guessing she didn't. Your husband made it worse by telling his friend that she didn't contact you, and she feels that makes her look bad.
I don't know whether she rejected your FB friend request or whether she's just not on FB that much. But all the other responses (or lack of them) should tell you she's just not interested.
When you say "he sent the money for the dress," do you mean your husband sent money to the friend, or that the friend sent it to you? I'm not sure what to do with that. Either way, I'm guessing the wife wants nothing to do with any of it. If they sent the money to you, I'd send it back. If you sent it to them, I'd write it off as a loss or a wedding gift.
Now, were you planning to take your children on this trip before the flower girl thing came up? It seems very expensive, and usually when people have a destination wedding, they are looking for a party that (frankly) most people don't attend because of cost and logistics. I think you're kind of stuck now, because it seems most sensible to leave your kids at home with Grandma or a sitter, but everyone (all 3 of you) have pushed so hard on this that I don't know what the bride expects. Emailing her again, or putting the men in charge of communications, seem like bad ideas too.
I don't think you should be leery of her, because I'm not sure she did any of this. I think she and her husband don't have great communication, or she just wanted her friend's daughter or her niece (whoever the original kid was) but didn't want to open it up to auditions and substitutions, which is what it turned into. Maybe she should have answered you, but then again, maybe she thinks her husband was handling it (and he wasn't, at least not to her satisfaction). I suppose you have to do something to say you're sorry it's gotten so out of hand and you think it's best if the kids bow out of this whole thing to make it easier for her.