As I thought about what I wrote in Why I'm smug about language mixing, I realised that there were some underlying principles to our decisions related to our children's language learning. I guess all multilingual families have been through some sort of similar thought process, although perhaps a little more consciously for non-native bilingualism, and less so for what one could call "involuntary" multilingualism which is sprung upon the family by the language situation they happen to find themselves in i.e. where the parents may speak different languages, and/or the community language is different to the home language(s).
What struck me as I wrote the "smug" post is that in our case we are not practicing multilingualism for its own sake, or for some cognitive benefits discussed by academics and eager parents, but to achieve some communicative goals specific to our situation. For me it comes down to one question: "What do I want my children to be able to do with their languages when they are grown-up?" - and then therefore out of my direct tutelage. My answer is that I would like them to be able to choose between living where they grew up (where French is spoken), or setting up in one or the other of their heritage countries (UK or Germany). Or of course going somewhere else altogether, but I can't plan for their future passions, whether for a particular language and culture or a person, at this point in time.
Providing them with the means to make that choice is what it's all about. So if a child doesn't actually use one of the languages they are exposed to during childhood, despite the parents' best efforts, I don't see the situation as irremediable. What has probably happened is that for the child, that language just doesn't seem relevant to the context they find themselves in. This is why parents of so-called "at risk bilinguals" have such a hard time: i.e. where only one parent speaks the minority language, and the other parent and all the rest of their day to day environment speaks another. Especially if the parent speaking the minority language also speaks, or at least understands, the dominant language, which is often the case (I figure the parents have to communicate too, right?). It's going to take some effort on the part of the child to ramp up to that minority language, and kids just don't have that sense of altruism that would allow them to "do it for Mummy/Daddy". Nor does the argument "Just think, when you're bigger you'll be able to read Cervantes in the original!" cut much ice with a 3-year old. There are of course other ways of motivating children to use the minority language - I just read smashed pea's great post How we became an opol family on how they pulled their kids round to using more German in an English-speaking environment, but you can see how much linguistic, emotional, parental and logistical effort that cost (hats off to them for getting there!).
But even in cases where it doesn't work, I think parents who continue to expose the child to another language whether they actually use that language or not, are still giving their children the potential to develop that language later on, should they find themselves in an environment where it suddenly becomes a whole lot more meaningful to them. Such as moving somewhere where the language is spoken, or falling in love with someone who speaks that language. Or even being motivated by a really good teacher. Because the advantages of passive knowledge of a language should not be underestimated, as a basis for future communication. I think the key with children though has to be the continued exposure, especially if they never develop the active mastery of the language. If you stop, it will go away. Everybody knows stories of children who spoke fluent x when they were 4 because they lived in x-land, but when they moved back to z-land they completely forgot the language, and didn't even have any advantage over the other kids when it was taught at school.
But I should stop pontificating and get back to my point, which was about being an enabling multilingual parent. My enabling goals for the children are, admittedly, pretty high. For them to be able to function as "local" adults in one of three places, they need to have GOOD literacy skills. Face it, if someone can't put together a decent CV, or email to the boss, or love letter, they may well have difficulties achieving some of their goals in life, whether it's getting a job, playing office politics, or getting it together with the love of their life. These things are hard enough to do anyway, without having a language handicap too! So this is why it's really important for me to aim high with my kids' reading and writing in all of their languages.