”I see your point of view but I will repeat myself here, which won't go down well with Flowery.” 
”Genuine question here, do you actually think that casting the child as Britney/Lizzie/Naomi will have more of an impact in fighting casual sexism?“ No I don’t “actually think” that. I never said that. I think cast a girl as the Innkeeper, as I’ve said several times.
”How am I completely wrong in thinking that on the contrary , keeping the " innkeeper's wife" and pointing out to children that this wouldn't be acceptable now is actually a perfectly valid way to bring up the topic of casual sexism?”
You’re not completely wrong. It’s just far less likely to happen/be effective, and isn’t the reason most people on this thread think it shouldn’t be changed, other than you.
”Although this question is more for Flowery who is the vocal one about this suggestion.” 
”No one necessarily has the answer, but I am genuinely asking, which of the two ( name vs IK wife with the issue pointed out) is the most effective way to address casual sexism.”
As I’ve said, several times, the most effective way is to cast a girl as the Innkeeper.
The OP has already agreed to have a word with DD which is great.
”Flowery you seem convinced that teachers won't point out the issue. What makes you so sure about this? Surely at least some of them will!”
Where are you getting that from? Most of the teachers I know and have worked with would absolutely see this type of thing as an opportunity to undermine stereotypes and not automatically stick with whatever has been done before. But there are at least a couple of teachers on this thread who clearly wouldn’t, and seems like the OP’s DD’s teacher is also on that list. I’ve no idea why you think I think most teachers wouldn’t. I’m very clearly talking about ones who wouldn’t, not assuming none will.
Good grief.