First, most white childrens' impressions of blacks come from interacting with black children--not all of whom, unfortunately, are at any one time candidates for sainthood any more than white kids are. And of those which are distinctly not candidates for sainthood, most have had racist inculcation before ever they set foot in school. Any effective racist indoctrination to which a child, white or black, might be subject is pretty likely to occur before school, rather than after first grade. "Education" is not a magic bullet for eliminating racisim.Secondly, a first-grader is not going to have any context to digest the information about racism; that information standing alone is a half-truth. It is a half-truth because not only didn't enslaved blacks have full standing as voting citizens before 1860, not only didn't women have the vote until about 1920, but nearly all the ancestors of nearly all Americans, however white and male they were, had no voice in government. The truth of America is not the hole in the donut, it is the donut itself--the fact that equality before the law is vastly improved over the past. And that equality before the law had to be invented here.
The big picture of American freedom is glorious, and what we now see as the dark portions of the past merely illustrate how easy it was for things to be worse than they now are.
The big picture of American prosperity is glorious; if you are at the "poverty line" now you're more prosperous than my middle-class family was in 1950. In fact that prosperity compares with what the salary of the president of the United States would have bought two centuries ago.
To teach first graders to take the big picture for granted and focus only on the negative is manipulative. In fact, it is exactly what a journalist does, and you cannot learn history from a newspaper. Time enough to learn the details and caveats of history after you have some concept of the big picture.