My dc are growing up in a non English speaking country attending native language schools, so until fairly recently I had all the power [evil cackle] over their English vocabulary and pronunciation and they all spoke grammatically correct fairly neutral English with slightly northern vowels.
Since I've been a Rabenmutter (i.e. leaving them to their own devices more due to work and studying) they watch far too much American TV.
I hate, hate hate, with a furious but I know illogical passion, American prepositions. I hate "on accident" "on the weekend" "on Christmas", I hate, hate beyond words "I'm excited for" instead of "I'm excited about" because it means something different and dumbing down the language to reduce its use to convey meaning is not language evolving but devolving.
I correct "I'm excited for the holidays" - no you're bloody not, unless The holidays is somebody's name and you feel empathetic excitement on their behalf! I can't help it, it makes my blood boil, and fortunately it's become a joke and they are aware of exactly what the difference between about and for is, and now only do it deliberately to wind me up 


However I have to make a conscious effort to let the rest slide so as not to put them off speaking English!
I'm sad to see their lovely correct British English slipping into sloppy slapdash non specific American, but it is limited mainly to prepositions of time and doesn't extend to accent.
Picking up an accents and dialect because you actually live in the location is totally fine, but people (and it's not only kids and teens) who mimic accents/ grammar/ dialect they've no human, direct link to just sound out of place and odd.