I don't understand. In the previous sentences there is no "THE" in English translation: Coçuklar yetişkin değildir=Children are not adults Muşteriler bu satıcıdan nefret ediyor=Customers hate this vendor but in this Yolcular çok yorgunlar= "THE" travelers are very tired. Does it have something with the specific direct object. Because there is no difference in Turkish?
In English, when you use "the" in sentences like these, you are talking about a specific person or group of people. Without the "the", you describe general traits of the person or people.
"Children are small" = "Most children usually are small"
"The children are small" = "In this group of people, the children are all small (while the adults would not be)."