A friend of a friend adopted two Russian sisters (~4 y/o and ~9 y/o) way back in the 90s after the collapse of the USSR. The kids didn’t speak a lick of English and their new parents didn’t speak a lick of Russian. Less than six months after moving to the US, the kids were fluent in English. Your kids will be fine
I’d say the only thing you should keep an eye out for, in any language, is how good their grammar is. If they’re really struggling with it by 6th grade, hire someone to teach them correct grammar.
one thing i should've clarified in original cast is that we're not living in an Anglophone country at the moment, which adds another complication to the situation 😅