Childishness in this instance is deciding that politeness trumps doing the right thing.
Politeness tends to reinforce existing subconscious norms, which in practice often includes women not "getting a seat at the table" despite being at the table. If you're a disadvantaged demographic in that setting, politeness is your ally only to the extent that the majority will enforce things for you. But the problem is that if you're a woman/minority, there's a class of issues where social enforcement happens far too infrequently. You can't trust the system to actually look out for you.
As the demographic who has to play from behind, politeness as a mechanism is insufficient to look after your own interests.
Hubeny is a part of a fortunate group of people who follow a certain pattern: some degree of "brick walls be damned" determination, some degree of willful self-exceptionalism, and a lot of luck. It's a powerful combination and she has earned her success, but that combination of traits to succeed despite tilted odds is very far from the norm, and it's the norm that needs to shift. I can guarantee you that among Hubeny's peers - other women at the tops of their fields - they view politeness as a fickle ally at best because they are consistently working at its edges out of necessity.
If civility is really such a barrier to equality (or whoever perceives it so), why not take this to its logical conclusion as Femen has? Or if it can be conceded that a more nuanced approach is better, why assume the worst of the offender with direct social attack, if you're willing to admit already that the behavior might be subconscious and indirect?
Maybe I was raised in a different generation, but disrupting a staged event from the audience unless it's done to prevent clear and present danger is considered rude, and two rude behaviors (Holt's being the first) do not make etiquette, even if comes from a place of warranted indignation. That's what post-commentary media is for. It's not a workable system to praise everyone who gives into their angry instincts in the moment without regard to decorum. That's called childishness.
I'm sure someone will read that as an apology for Holt. Far from it. It's about finding a time and a place to voice one's objections, not losing control and exploiting the attention of a quiet crowd, and then self-styling one's self as a crusader who "created an opening" because one gets a few pats on the back for it, and far from showing any remorse about her timing, saying "please let me not be afraid to do this again ...and again ...and again Because it was scary." Do we really want a one-woman Westboro Baptist Church of anti-sexism disrupting every event that she has decided is "scary"?