Ex primary teacher here.
I will preface this by saying that I would prefer no homework except for reading but the homework you are referring to is also not my favourite (for different reasons perhaps), especially when, like pp have said, you have to give nine different options. It seemed to be particularly fashionable in international schools but seems to be increasingly common over here too.
One school I worked for in particular absolutely lived and breathed this open ended choice-based work. Every assessment for every subject was basically "demonstrate everything you've learned about x in any way you want" and then you'd spend hours and hours watching videos of children teaching a fake lesson about fractions to their teddy bears or explaining their lego model of a Victorian workhouse or performing an interpretive dance about life in Tudor times. While it sounds great in theory, marking it was so time-consuming and inconsistent.
I do think there's value in these extremely open ended activities but I don't think it works well as homework nor do I think it should be assessed.
One thing I would say to "my child isn't capable of doing this activity" is how low is your bar and can you lower it further and then lower it again? When you hear "song" you think rhyme, but I would be extremely surprised if your six year old's teacher is expecting anything even remotely rhyming. If your DS is capable of something resembling:
"Fire
So hot
Burning"
then they can do the homework. How well they do it is based on their ability. And that's what the teacher wants to see. There is nothing worse than marking 30 lots of homework clearly done by adults. There is no need to help them find rhyming words. If they are capable of just writing a few words somewhat related to fire, then arguably they have created a song. You can say anything in a song-song voice and it's a song. If they are capable of drawing a picture and copying down the title of a book, then they have created a book cover.
I once had a parent make a complaint about a topic where year 5 students had to draw a (poster) advertisement for a chocolate bar they had designed. The parent argued that it was way beyond a year 5's capabilities and that people work in marketing for years to learn those skills. As we spoke, it was clear that what he meant was that doing it WELL (from an adult perspective) is beyond the capabilities of a year 5 student.
Nobody is expecting it to be adult-level good. Nobody is expecting a 5 year old to write a top 40 hit. But within the craziness of the tic-tac-toe grid of open-ended homework options, in my opinion, one of the benefits the only benefit was that the children who were used to this approach became a lot more confident with giving literally anything a go and not worrying about whether it was good enough. I would argued that they tended to be a lot more confident in their own ideas and were more able to work independently. If they're capable of doing it to any level at all, they are capable of doing it. They end product naturally gets better as they progress.
So in summary, while I do agree with you, it's likely a school-wide homework policy and so I personally would just take the good out of it. Once they're used to it, and have the confidence to just give things a go, you don't have crying over addition sums that they can't do or endless spelling practice.
If at first your child needs help to do it independently, I would suggest prompts like this:
"Let's think about some words to describe fire. What is fire like?"
"Can you tell me about the fire of London?"
"Let's write down the words you're thinking of"
"Let's pick our favourite of these words and put them together"
I would not suggest help like this:
"Dire rhymes with fire. Dire means really bad. Yes it was really bad. Okay maybe we could say "everything was really dire". What do you think?"
When you help, do it with the aim of getting them to a point where they are confident enough to attempt things independently, no matter how bad what they come up with is. Don't help them with the idea of getting a good end product. Once they are at that point, as they start getting older, you can always start encouraging them to check, edit, improve etc. if you feel it's appropriate. But for now, the best help you can give them is to teach them how to sit down to a task like "write a song" and come up with anything at all.