I just don’t even know how you would argue that they aren’t making art. What purer form of artistic endeavor could you name than a child being creative for its own sake? Things like technical skill, novelty, complexity, etc. are qualities that art has, but they have nothing to do with the definition itself. If a child creates something with artistic intent, that’s art. Arguing otherwise is navel-gazing prescriptivism, the same basic argument as Roman statue fetishism and just as tedious.