I’ve read all your posts OP but not all of the replies so I’m sorry if this is repetitive.
i get the impression that you see singing a song to mark a transition as immature, but repectfully I think this concern is misplaced. Music is used to mark transition in many adult media like film, tv, news programmes.
secondly, I don’t think you maybe appreciate how loud a classroom of 30+ 8 year olds can be, and starting a song can be a way of getting their attention without shouting at them. Surely this is preferable?
no doubt there will be schools around who take a more sombre approach, but given how fast kids are having to grow up thanks to the internet, culture etc etc, surely a bit of light hearted approach here and there is a nice thing?
its fair enough if your child finds it a bit naff, but as you said its not hurting anyone, so where’s the harm?
complaining to the head is unlikely to achieve anything except making probably an already stressed and overloaded teacher feel criticised to be honest.