Parents who don't like what their school libraries stock should lobby their school board, vote for school board members who share their views, and maybe even run for school board. There's a whole nice, approachable, local body to deal with all this, basically everywhere in the US. It's arguably one of the closest-to-the-people and easiest-for-constituents-to-influence governing bodies that exists in this country.

[EDIT] "Wait, so why's that not what the outlets and politicians who are complaining about this, are advising as the right way to approach this?" because the entire point of making this non-issue an issue in the first place was to boost state and national level candidates, not to do anything useful or appropriate—a state governor can't grandstand about "get mad, and go... um... vote in your school board elections!" in a way that'll be effective at building their national profile or driving broader GOTV for their side.