I absolutely share your views, François! It is unfair to "bilinguals-in-the-making" children, to go and complain about them not speaking either of their languages well. I believe that this is, as you rightly point out, a matter of maturity. As the toddler matures to child and then to youngster, conscience develops about code switching and simultaneously this mechanism of sorts becomes less and less frequent. Apart from which of the codes is dominant, there are other factors to consider in language intermingling - not in children but rather in full-fledged adult bilinguals - which are the geographical factor (people living in border areas between countries using different languages), professional (as occurs for instance in the case of linguists or translators, and also with business or trade jargon), technological (technical terminology and dominance of English) and last but not least, social (snobism) or personal (affectation, trendy buzzwords) factors. I am sure that you must surely have earmarked some of these topics for research and I already anticipate your articles on these fascinating aspects of linguistic interference in bilinguals.
All the best!