What is the advantage that a small child learning two languages will miss, but becomes apparent very quickly later? The fact that communicating a juicy bit of gossip or highly controversial opinion in public incognito-like may be possible in the other tongue.
NO! I do not encourage gossip or disparaging remarks under any circumstances, but it may be seen as a sufficient motivation for a kid to keep up their language skills in the target language.
Like this day, when it may have been whispered; "Papa, j'ai failli pleurer quand j'ai donné une souris à manger à l'hibou." or "Papa, I almost cried when I gave the owl a mouse to eat." The young naturalist did not want to be seen as less than capable, but in her tender heart something else was happening that she felt free to express only in another language.
When vocabulary skills are limited in the beginning stages of a language, it is great to have a few key words that on their own send a signal to the partner/family member that there is an urgent need to communicate secretly, or an event to observe. "La crise!" in French translates to "Holy crud!" and is just the word for many an occasion. "Trop fatigué" can mean "it's time to head home, find a good excuse." In our family, this phrase is used, for some odd reason, in French combined with Pig-Latin. I could not for the life of me, tell you when or where that began. I must have taught my husband Pig-Latin one day so that we could talk without the accusation that we were using English in front of the family.
Again, the question will arise of good manners. I do not advocate for speaking in another language right in front of someone, unless it is while interpreting for two parties who cannot communicate, but when there could be an eavesdropper lurking around the corner, it can be handy.
Youth, and the less young, must be wary, however, of their perpetual habit of conversing in a mix of two languages. When your comment includes an unpleasant term that you habitually say in English in the midst of your French, you will be the object of glares and scorn and suspicion. "T'as vu, là-bas? On dirait un big booger sur sa chemise." Yes, the "booger" will get you into trouble, especially when combined with the English adjective that seems to flow in relation to the object.
But if it means they are making an effort to put into practice their second language...it is sort of fun, isn't it?