Translation:The Korean children are not playing by these trees, but by those.
For me it depends on the rest of the sentence. If I use "these ones," I'll follow it with "those ones." If I say "these nouns," I tend to just use "those." For example: "I don't want these ones, i want those ones," versus "I climbed these trees, not those." Also, when you talk about being educated in "standard english," you may want to be more specific. British and American English have alot of differences. I don't know if it matters in this specific case, but can in others.