I find that generally descriptions are intended to differentiate a character or group from those around them. It’s difference that makes us unique and it’s that uniqueness that is usually described. To go with the above example, if I read a description of a “group of black kids sat around a tree”, I assume they are described that way because they are in an environment where they stand out. Similarly, I wouldn’t be surprised to read “a group of white kids sat around a tree” in a setting where people are primarily non-white. If a book is set in a primarily black neighborhood, I wouldn’t expect kids that fit in there to be described as black at all. “A group of kids sat around a tree” would be more likely because they fit within the environment and therefore don’t need to be described as standing out. Therefore assuming “A group of kids sat around a tree” is white only works when the setting is one where you would expect white kids to be.