"I'd argue that getting kids excited about what they're writing, and content, is more important than SPaG at that age. Then get down to SPaG, if you absolutely have to."
I'd argue that NOT correcting errors as soon as they're spotted and teaching the CORRECT spelling or grammar means the WRONG spelling or usage becomes ingrained in the child's mind and harder to correct.
"If you can communicate ideas effectively, does it really matter if they're perfectly presented?" Because poor spag = poor communication. See it on here all the time, posts that are IMPOSSIBLE to understand because they're so poorly worded, even to the point the op then gets incorrect advice that offends them because respondents were UNABLE to understand the issue.
"Totally - unless SEN, Dyslexic etc." Of course, and evidence can be provided if that's the case and reasonable adjustments made. But that doesn't mean NT children shouldn't be expected to ALSO work to the best of their ability and have their spag correct.
"But surely when learning a subject you have to learn the rules.
Spelling and grammar are the scaffolding on which English is built." Exactly!
"I don't have kids so I'm unfamiliar with this sort of child competitive landscape, but are there similar allowances for children with dyscalculia in maths competitions? And children with physical disabilities in sports competitions?" As someone with a child with a physical disability I would say no.