Assuming these kids are in the U.S., then you omitting the topic from their 4th grade class would only omit a tiny portion of what they encounter in their lives. If my 4th grade teacher omitted something from history class, I don't think it's possible that I've never learned about it (assuming it has some significance).
Coming to the US is what made me encounter racism. American society really is broken from within when third world countries can get around this but America can't.
I spent three years living and working in a third world county where I “stuck out like a sore thumb”. Most people I met assumed I was rich, despite living and working on the equivalent of a local secondary school teacher’s salary. And this was still a hell of a lot more than most people in the country were making, but my local counterparts did not have to constantly deny requests for money and things from strangers.
Perhaps a more apt comparison to racism in the US would be sexism in the country I worked in. Men would cat call women constantly to ask for (or demand) sex and marriage. This was just an accepted and normal part of the culture there. It was crazy.
Maybe other countries score lower in sexism. (btw America does too, we all know that :) ) However, it still doesn't take away the fact that the extent of racism in America is far too high for a first world country.
As much as it pains me to say this, at a societal level, I don't see any difference between my third world country of birth and America. The only difference between my home country and USA is interstate highways.
Unfortunately, that has been my experience. A lot of seemingly well-intentioned Americans refuse to accept their follies. They are completely unaware/unwilling to admit that the rest of the world is watching what's happening in America.
This is possibly due to the ingrained, "American exceptionalism" belief but I am willing to be proven wrong.
As a comparison, see this Brazilian admitting flaws in Brazil: https://news.ycombinator.com/item?id=17735229
But Americans admitting racism? Nope.
But by 4th grade I'd be teaching about slavery, racism, etc, and the kids would be then learning about a concept that was utterly nonsensical to the non-racism-indoctrinated kids. So I'm wondering what would happen if I never taught about racism at all.