My husband's parents only speak French. What has always struck me as one of the more subtle aspects of translating works in English to these other languages is that the translator would have to decide how characters would address each other, and if this would change in the course of the story. There's definitely something I don't understand there. If you were to read Balzac or Hugo, that is what you would find. I haven't read Little Dorritt in a long, long time, so I just might be misinterpreting here.
Guys have to show some distance to prove they can be on their own.
Tu for familiarity, vous to teach the kids some distance and independence. That's where I'm not too sure. 2 people that have know each other for a long time would say tu. I don't know about Arthur. Rigaud with the tu says his superiority over Cavalletto.