We're coming up to our 3yrs here, and all my kids accents have changed. My oldest at 17 is the worst, I know they have to adjust for school but she sounds more aussie than her aussie bf at times, which is very bizzare!!
my 10yr old has a slight shift in accent slips in and out some words sound aussie but mostly still very Welsh.
My youngest however who is now 4 is full blown aussie accent as he's been attending child care and kindy, and he tells me that I don,t talk properly and tries to correct the way I say things. cheeky monkey.