My experience with immigrant families (including my own) is that children almost never achieve full fluency in their parents’ mother tongue. They may be able to communicate effectively with their parents and reach a reasonable proficiency in the language, but they will consistently retain a noticeable accent. I have observed this with Portuguese (myself and several friends), as well as French, English, and German.

If you want a child to develop true fluency in English, send them to an English-speaking country before their teenage years. I have seen children up to about age 15 lose their accent this way. However, those who moved after 18 generally retained their accent and never reached full fluency.