What is irking people is not the achievement, but the meticulous documentation and flagrant promotion of the achievement. Guinness Book of Records didn't invite themselves you know. They also don't come for free. Typically you need to pay a hefty fee to come and document your record AFAIK (another sign of privilege).
More kids may have similar achievements but their parents never sought publicity photo shoots for them. Exposing kids to this kind of publicity may be bad for them down the line. May make them feel like a failure if they aren't able to pull another newsworthy stunt in the future.
