DS has a music box that is meant to play 'O Christmas Tree', am I wrong...

[MarionCole]

to teach him the words to The Red Flag?

no you are quite right.

The Internationale is also a good one; so is The Marseillaise.

All rousing songs with a powerful heritage.

Whether you should bring him up to be a socialist, well that's up to you...

No - teach him the Red Flag, my dad taught it to me when I was little

it's a bit gruesome though isn't it?
(can only think of first two lines)

Yes a bit gruesome - all the stuff about martyrs blood, and gallows etc. appealed to a rather over dramatic child, with a very left wing dad.

I sing the football versions of nursery rhymes to my son.

My dad taught me the red flag and several rude versions as well

The Marseilleise is incredibly gory - all about watering the fields with the blood of yr enemies. Wish they'd bring a bit of that into the general election

It only plays the chorus, so I don't need to worry about the gory verses.