MFL = Music For Languages

I'm convinced there's a connection between the ear for music and the ear for language. If you  can detect a slight off- note or bumpy rhythm while listening to the kids' violin or piano practice, it's quite likely you'll find it easier to discriminate between the vowel sounds and stress patterns of another tongue.  If you can't tell the difference between a triangle and a tuba,  read no further - this is not for you.

As children learn their native language, songs and rhymes are important.  The tunes,  rhythms and  repetition of simple words and phrases implant themselves into the brains of  infants who rapidly become accustomed to the  cadences of the mother tongue.

If you can catch children before they become silenced (or worse) by hormones and inhibitions, well-chosen songs can be a useful learning tool for a second language.  Songs and rhymes provide effortless repetition of key words and phrases and painless teaching of grammar points.  The best tunes are ear-worms: aiding recall and implanting  linguistic rhythms into the subconscious. 

However, if you decide to use song in the FL classroom, you need to be selective.  Children learning a foreign language are not in an immersive situation, so nothing is  reinforced by the  language they hear around them all day.  You need to choose songs which teach, rehearse and continue to repeat specific vocabulary and language elements.

Adapting English Songs
You have to be careful about the rhythms of the language you are teaching.  Some teachers are prepared to have a go at fitting foreign words to English tunes.  So far, so commendable.  Germanic languages follow the same stress patterns as English, so it's usually possible  to do it successfully. The following, for example, works perfectly well to the tune of London's Burning, :        
        Eine Katze, Eine Katze
​        Eine Schlange, Eine Schlange
        Ein Fisch, Ein Fisch
        Ein Kaninchen, Ein Kaninchen.

And this one to the tune of Nuts in May:
        Hier ist ein Hund und ein Papagei
         Hier ist ein Fisch und ein Elefant
         Hier is ein Bär und ein Schmetterling
         Hier ist ein Pferd und ein Krokodil.

The same is true of some Latin-based languages.  Italian and Spanish usually have the stress on the penultimate syllable, as in English, so translating, writing and adapting are relatively straight forward.  

French is another matter altogether.  The stress is either equal throughout the word, or falls decidedly on the final syllable.  So tunes written for English lyrics will not accommodate French words comfortably, and if you try to make them do that, you run the risk of teaching inappropriate word and sentence stress.  The following are monstrosities to be avoided because the English tune plays havoc with the French stress patterns:
(Ten Little Indians - inappropriate stresses marked)
         Un petit, deux petits, trois petits lapins
         Quatre petits, cinq petits, six petits lapins   
         Sept  petits, cinq petits, six petits lapins
         Dix petits lapins sautent

(We Three Kings of Orient Are - inappropriate stresses marked)
         Les trois rois regardent l'étoile
         Les trois rois regardent l'étoile
         Regardez, regardez   
        Des cadeaux pour l'enfant Jésus.

(Jingle Bells - inappropriate stresses marked)
         Janvier, février
         Mars avril et mai
         Juin, juillet
         Août, septembre
         Octobre, novembre, décembre

French is the language of choice for most British primary schools, but it is the most difficult of the European languages to pronounce.  Not many primary schools  have the luxury of language specialists or native speakers to model pronunciation accurately, so it is all the more important to get audio resources right. But there are alternatives.

Using Authentic French Songs
If you choose authentic French songs to use in the classroom, you can be pretty sure that the rhythms of the language will be accurate.  There are plenty of recordings of traditional French songs which provide good models of pronunciation. But it's not all good news.  Beyond the pronunciation, they serve little linguistic purpose.  The vocabulary is not particularly valuable, the structures are too complicated for beginners, and many contain outdated or literary turns of phrase:
       II court, il court le furet
       Il court, il court le furet,        
       Le furet des bois, mesdames;
       Il court, il court le furet,
       Le furet du bois joli.
       Il est passé par ici,
       Il repassera par là.

The English translation goes something like this:
       He runs, the ferret, he runs
        He runs, the ferret, he runs
        He runs, the ferret, he runs
        The ferret of the woods, ladies
        He runs, the ferret, he runs
        The ferret of the pretty wood
        He passed by this way
        He'll pass again that way

For the rest of the lyrics, in French and in English, go to Mama Lisa's World Nursery Rhymes, where you'll also find the sheet music and a very good recording of the song by a native speaker.  
        
Charming as it is, it's not much use in the primary languages classroom except for its novelty value.  The same goes for other traditional French songs and nursery rhymes: Quand trois poules vont au champ (When three hens go to the field), ​J'ai perdu le do de ma clarinet (I've lost the C on my clarinet), Alouette (Lark), Sur le pont d'Avignon (On the bridge of Avignon), ​Savez-vous planter les choux? (Do you know how to plant cabbages?), ​Un, deux trois, nous irons aux bois (One, two three, we'll go to the woods), and - my all-time personal favourite - J'ai du bon tabac dans ma tabatièrè (I've got some good tobacco in my tobacco pouch)!

On the other hand, because these melodies are so catchy, and written for the French language, they can be used to advantage when we replace  original words with the key words and phrases we want to teach.

Replacing Original French Lyrics with Useful Language for the FL Classroom
It's quite easy.  You just keep your teaching point firmly in  mind, and glue that together with useful words and phrases.  Not a woodland ferret to be seen...

I have unearthed several that I wrote a few years ago when  teaching French to KS1 pupils. 
       ​Un, deux, trois - viens avec moi   (to the tune of Un, deux trois, nous irons aux bois )
       Un deux trois, viens avec moi      
       Quatre, cinq six, on va à Nice
       Sept, huit, neuf, j'ai un vélo neuf
       Dix, onze douze, allons à Toulouse.  

      J'ai un petit frère    (J'ai du bon tabac dans ma tabatière)
      J'ai un petit frère qui s'appelle Émile.
      J'ai une grande sœur qui s'appelle Cécile
      J'ai un petit chien – il est tout mignon.
      Je n'ai pas de chat, mais j'ai un poisson.
       Viens, je te présente ma famille
       Mon père, ma mère, Émile et Cécile.
      J'ai un petit frère qui s'appelle Émile
      J'ai une grande sœur qui s'appelle Cécile.

The final option, of course, is to invent both your own lyrics and melodies.  It's not that difficult provided you start with the rhythm of the phrase you want to teach, and not with a melody.  That way you'll have more chance of writing a tune that follows the stress of the target language.  

Good luck!