Was the black kid being a part of the team unrealistic?
This movie is set in 1962 in LA county, and racial tensions were still very high at that time and in that area. I can imagine several ignorant, hot-headed white kids (like some in the group are) being racist, or the parents of the kids not wanting their kids hanging out with a black kid. Also, it was possibly unrealistic during that time that Kenny would be allowed in a public pool.
I know by 1962 there were several great black baseball players like Willie Mays and Hank Aaron that the kids were probably fans of, so those kids very well could've gotten comfortable with black people by then. Also, Kenny was a great pitcher so the kids were probably very happy to have him on their team. But it's still something to think about.