Typically a child picks up the accent of his/her peers and culture, not his/her parents.
Based on a sample size of two (my daughters) I can attest to their both
speaking with wonderful northern English accents at the ages of four
and nine, just prior to our moving to the US. Now, at 16 and 21, they
are never mistaken for being English and have to actually work hard to
mimic an English accent!
I should point out that northern English words such as "fettler" or
"daft" are, in fact, pronounced much closer to the real thing because
these are words they have picked up from their parents and have no US
equivalent. But to all intents and purposes, their accent is pure
Ohioan!
Siggy