Who would you cast in a modern remake?

Take it from someone who works in a school, that is absolutely not true (and a very cynical view of the world). There is definitely a difference in clothing worn by children and clothing worn by adults, and people do not treat 12 year olds like adults. In fact, most educators would tell you there is a crisis of infantalization going on in most schools. Parents baby their children well past a reasonable age nowadays.

Still, you're right, you could never do a remake with the character at the same age. Sure the situation is convoluted, but I'm sure modern day writers could figure out a way to smooth it out so that it's more believable to a modern audience. No, the reason you can't do "The Major and the Minor" now is because it was key that the Major *was* attracted to the Minor, even though he creeped himself out. The scene in his office is amusing because he notices how attractive she is. The Major and the Minor was fine THEN, in 1942, and we can watch that movie with innocent eyes as was intended. It's amusing because a grown man wouldn't knowingly pursue a child that age! Ha-Ha! What a dilemma!
However, we can't make the same movie now and unlearn what we know about the world. It would be on everyone's mind. The very fact of finding her attractive in her 12 year old state would disturb people. That will make the audience think of "Lolita", and when you're trying to get people to enjoy themselves at a light comedy, that is not a story you want popping into their heads.