Who gets to pick and choose the kids' specialization ?

I don't agree with the premise (that early specialization is good), but setting that aside deciding what a 4th grader should focus on for the rest of their life feels like an impossible task.

You can't account for whether they'll like the more advanced subjects (i.e. they like 4th grade literature class but will bail at languistic analysis) and have no idea how they'll change even 3 years later as they hit puberty.