It's interesting that this is being painted as new (but then, there always has to be SOMETHING to publish, and attacking parents is usually a guaranteed click-source).
Sociologists have been following the same basic thing pretty much since sociology has been a thing: the children of the poor are highly independent and have to be able to entertain themselves; the children of the upper classes are hugely scheduled and instructed; middle-class kids get a mix.
Back in the day it would have been boarding school and polo; today those kids mostly live at home, so it's lessons of all kinds, but the basics are the same.
That said: I'm grateful as hell for the independent parts of my own middle-class upbringing. I spent huge chunks of my childhood up a tree, while my kid sister, who was born when my parents were already doing much, much better, was very heavily scheduled (and loved it, and pursued the arts--so she wasn't thrown into Wall Street or something).
I think the Mustachian angle on this kind of thing is parents pushing their kids in this way often also push for the most expensive education, leaving their kids saddled with debt, and thereby forced into jobs they may hate but need to pay those debts back. But violin lessons, meh. I wish I'd been forced to learn a few more things when my brain was fresher, to be honest. It's not exactly the worst thing in the world to be exposed to more education, and if the parents want to bring those costs into their lives, so be it, I guess.