My mobile completely cut off my response so this is part two!
I moved to England with my four-year-old fully American son. His father is from Houston and is American and we divorced when our son was two-years-old. I moved with our son to England to marry my English fiance (that was 19 years ago yesterday, yay, us!).
In the beginning, my son had a very strong American accent and his favorite expression was, "Check it out, man!" and found that most people found him amusing. He started nursery school in two months of our arrival and almost overnight he caught a somewhat posh Berkshire accent (we lived in Wokingham). My husband is from Wigan, in the northwest so it was definitely an accent brought on by his environment.
It was indeed quite odd when we visited his father back in Houston because our son was the same child but with a completely different voice. I know that his paternal grandparents found it difficult to understand him since they were not confident with the English language to begin with (they are from Mexico) and this young boy with an English accent was a real test to their grasp of English.
We did finally move back to the US almost ten years later. In addition to my 14 year-old, I had two English-born children who were then seven and three. My oldest child happily went to school to find out that he was completely ostracized for his accent and his lack of social references that the other kids were tuned into. He struggled in many lessons such as spelling, geography and history. It was probably a month before he made the decision to completely change his accent. In fact, he completely decided to reject anything relating to his life in England.
My children are 23, 17 and 14 now. My oldest child has a neutral American accent but when he's in my home (he's grown with his own little family), he relaxes so I do catch his English accent and his use of English words for every day things. My middle son has a slight English accent still but uses English words frequently. My daughter lost her accent the day she arrived in the USA. She has a Houston accent so it's a little bit of a Texas twang. One thing about my three is that they are great at drama at school because they can always play an English character with ease!