He could've had some back in Europe, but I don't foresee them giving him any type of accent. They could, but I doubt it. Or the kids could've been put up for adoption (which would certainly be appropriate) and then adopted by Americans almost immediately. Who knows? They may not explain how he and his hinted-at sister wound up where they are if they do acknowledge the familial bonds with Magneto, but all it takes is the imagination!
As for why they wouldn't have appeared in the films that take place after FC and DoFP's 1973, they were never cut out to be villains. That's what led to their leave from the Brotherhood and their eventual joining with the Avengers. But I always imagined - should Magneto be his father in the film - that Wolverine might explain to the younger Magneto that the Sentinels kill Pietro and Wanda in the future as a means to ground Erik and encourage him to work with them to stop the "war before it ever happens." Which might also explain why Pietro's role is rather limited and why he may not be featured in the climax... his father doesn't want to take the chance. All speculation, of course!