Formal night has been debated at length countless times on this board.
Regardless of your opinion of what is appropriate the fact of the matter is some people like to dress up and others do not.
For some, formal night means being "dressed to the nines" while others will show up in very casual attire.
I have never worn a tuxedo and I certainly don't want to start while on vacation so I have to say that I side with the casual attire folks.
That being said, I will tell you that there are a few options on formal night to have a family portrait taken with a nice
DCL backdrop.
Since a
Disney cruise doesn't come along that often for most and can be a once-in-a-lifetime experience for some I would explain to your DD that you want her to put on a nice dress for about an hour and a half, maybe two hours so Mommy can have a picture and a night to remember the rest of her life.
I wish you luck,
Jeff