Well, that’s an excellent question. Not terribly informative but fun.
The first year we were here, we were wondering if our daughter would ever sound less American. Not that there’s a problem with that, but in a household of mixed languages (American/German), with her spending the better part of the day surrounded by British English, we would have thought that transition would happen quite quickly. It didn’t.
I’m sure the variation between individual children is substantial, and picking up accents is highly dependent on the age of the child. Ours had just turned 4 when we moved to London.
Now, at age 6, she sounds neither properly American, nor properly British. She’s at the height of her Britishness when she practices reading – the phonics system to learn the sounds of letters and letter combinations has left a deep mark on her pronunciation. As have classmates who taught her new words and concepts (“Daddy, who is Princess Layer?”). And, funniest of all, the round-about British way of saying things (“Mummy, I might like to use that for my art project.”).
The photo, however, shows how she (blond hair) felt after Dad (brown hair) told her off (I can’t remember what for) and she stomped into her room to express how she felt. Note the zigzaggy tongue, sticking out nicely. Universal language.