How about: çocuklar gidiyorler, i.e. Adding the plural suffix to the predicate?
It's okay, but it's optional since it's obvious that the subject is plural the children.
Çocuklar gidiyorLAR. Pay attention! İf there is e,i,ö before the plural suffix, you must use "ler", for the others (a,ı,o) "lar".
why its not gidiyorlar
Why not gidiyorlar?