Weird scene

Not necessarily. She was supposed to become a prostitute, so she had to learn her future job. Those days school was not as obligatory as today, and after few (mostly four) classes kids who weren't going to continue school education, but didn't stay at home either (e.g. on farms), had to learn some profession. They usually went to some craftsman like tailor, shoemaker, carpenter etc, and stayed continously, often even slept there. First days and weeks kids were watching, then giving a hand like passing tools, also cleaning the workhouses, after few months they were able to do some basic job themselves, and their education continued for several years until they were able to become complete professional. As Violet lived in a brothel she was supposed to become a trained prostitute, so she had to learn gradually what she needed for her future profession. Maybe weird, but logical if you think a bit about it. We don't know what stage of education was she in when these scene happens.

Some probably do. And majority doesn't. And it doesn't, statistically, matter if a child is nude or in armor, playind a prostitute or an angel. It is firs of all the matter of how (un)comfortable a child feels in front of the camera. If they can feel comfortable or even relaxed, most scenes won't make a problem; otherwise it really doesn't matter what role they have to play. Of course, unless they really don't like something or are afraid (as if a child hates apples and the director and the screenplay insist that a character has to eat one, or a child is afraid of dogs and the character has a dog and plays with the pet). So it is possible that some kid actors are uncomfortable to be nude, another to ride a horse, and some others to be in a boat or on the skyscraper. However, this is a job of parents who can be the only ones to know well enough what are the limits of their kid's acting. And to chose proper roles nstead of forcing them. There would always be another kid actor ready to ride a horse, to be nude or to climb a tree. Respecting this would reduce almost to zero any need for later therapies.