It's not just children with disabilities. Organizations like Saga have reported that elderly patients, even those with dementia or quite severely ill, so it was questionable if they understood, had DNRs added to their files. My father fell over and was in hospital for 24 hours for observation. When my mum checked he had DNR added to his file, and no recollection of anyone asking him.
Luckily my mum is his carer with permission to check his records, so she was able to get it corrected. But how many people have had this done without their families knowing?
Anyone with a vulnerable operation in their care should check this if they've been into hospital. It seems to be a widespread and deliberate policy.