One of the notable details of Murmur of the Heart is the fact that Luarent's mother is not French but Italian. This would de-focus the controversy from the French. In addition to that, I think there is another similar ethnic distinction made in the film. That is regarding his little blonde friend, who passed him a note to start a friendship. It is fairly apparent that that boy looks Germanic, not French. His story name is Michel, which is supposedly German (http://en.wikipedia.org/wiki/Michel_(name)), and the actor's name is also German, Eric Walter. http://www.boyactors.org.uk/actors/777.jpg (The other (1951) movie attributed to him is an apparent error.)
This again de-focuses any possible mores inferences away from the French and shares them with other Europeans.
Anyway, as a basic distinction, French, Italian, German and so on are not ethnic categories, they are not about race, but about nationality. Back in the days of the Great War people talked about the 'German race', the 'French race' and so on. Books and newspapers and magazines published at that time were full of talk like that.
But the thing is, while there may be a more or less distinct French or Italian or German culture, it is quite errant to assign any overt racial attributes to people of these nationalities.
There has always been co-mingling and movement of Europeans from one part of the continent to another. This even before the Great Migrations of the Peoples in the 4th & 5th centuries CE. There is nothing much unusual for people of all nationalities and origins to be living in other countries, certainly not in 1950s France.
So, while the remarks are a good exercise in attentive looking, I think the nationalities have little to do with the story. Especially with Michel.
Perhaps it's not ethnic in the racial sense, but Europeans are very conscious of their national cultural differences, and assumed character, or at least psychological differences.
I had the distinct impression that Malle wanted to avoid the appearance of commenting on the French psyche. Why else would he bother with the story weight of the mother being Italian, so Laurent being half Italian, whom she calls by the name Renzino. According to this wikipedia article, Renzino is an Italian variation of Laurent. http://de.wikipedia.org/wiki/Laurentius
And the blond boy was clearly nordic, and he did take the initiative, both in meeting in the first place, and sharing the tent.
The story doesn't concern itself with nationality consciousness, but Malle does appear to be covering himself, and spreading the free-spiritedness around beyond the French.
Perhaps the mother being Italian is supposed to indicate that her temperament is a bit more fiery, more impassioned than the French temperament? Though it's all a question of point of view since I'm certain Germans and Dutch and Anglo-Saxons certainly consider the French to be temperamental and emotional. Maybe it's a way of excusing the mother being less conventional than the other grown-ups in the movie.
But it's certainly not racial. By the time the movie was made, any kind of thinking in Europe in those terms was long since abandoned.
The blond boy, Michel, while being blond, doesn't show any other kind of characteristics of being Nordic or Germanic that I can see. His name is as French as could be and he might as well have been of Belgian origin at some point. When I was in school in Belgium in the decades before the movie was made, but a decade after the movie setting, there were always blond boys in the classes and this was nothing special or out of the ordinary. And Belgians have always moved to France readily, even Dutch speaking Belgian families and French have always accepted Belgians as being part of the greater French cultural world ...
But the Belgian angle is just supposition on my part. Though it would be more likely than a German living in 1954 France. From one of Malle's other movies - Au Revoir les Enfants - I have the impression he doesn't much care for Germany and still has strong resentments - rightly so I'd say.
Anyway, this is a movie that is so in keeping with how I experienced school in Belgium a decade later, just like in the movie in a boys only religious run institution with so many of the attending attitudes and way of treating pupils.