Character mistake: The Sheffields are going to Miami but when they are there, Maxwell mentions having gone to Disney World on the same trip. Disney World is not in Miami, it's in Orlando.
Continuity mistake: When Sylvia comes in and wakes Fran, she puts her hands on her Hips when she says "grow old with dignity", and when the shot changes her hands are at her sides.
Continuity mistake: When Maxwell shakes Fran awake the first time (in her dream), his hands are on her shoulders. When the shot changes his hands are suddenly on her legs.
Continuity mistake: While Fran and Maxwell are changing the baby in the kitchen, the jar of baby food moves from next to the baby's head to next to his feet between shots.
Continuity mistake: When Fran is trying on her shoes in the shoe shop, her legs are crossed. Then in the next shot they are instantly uncrossed and she stands up.
Continuity mistake: When Fran and Val are in the sports bar in the beginning, the popcorn in their hair changes between shots. This is especially obvious on Val.
Continuity mistake: In the dining room during breakfast, while Fran is talking, Niles starts to turn away from the window towards Fran with a teapot in his hands. When the shot changes he is facing the window again and turns around with the teapot a second time.
Answer: Children's brains and language skills are still developing at that age and they adapt to the environment they live in. My former boss was born in England and moved to the US at about eight years old. She completely lost her British accent by her teens, even though her parents still spoke as typical English citizens. A Japanese co-worker and his wife, also Japanese, spoke English as their second language. Their two children learned both English and Japanese simultaneously while growing up and spoke each language with the appropriate accent.
raywest ★