These are not children. They are adults who know that acting childish is their means to establish supremacy.
You're talking about the rioters and protesters in the 1960s. You're talking about the NAACP and the ACLU and the CBC. You're talking about the SDS and the Black Panthers ... even Saint Martin Luther King.
Trump is a negotiator. One tactic is to deny the other side power by ignoring their demands...anyone with a three year old is familiar with this.
Think about it, someone calls you "Racist!" and you reply, "That's terrible, I'm so ashamed and sorry, what can I do to make it up to you?!" and you've just handed them all the power in the negotiation.
However, if (like Trump) you say, "I don't care.", then that takes all of the wind out of their sails. The only thing they can do is yell "RACIST!!" louder, in hopes that they annoy you into action....
....which is the point of discourse that we're at now, nationally. It's bad enough so that even low-intelligence voters are seeing the ridiculousness of it. Should it keep up - and so long as Trump doesn't appease those who are yelling - 2020 will be a shoe-in for him.