Why could we use yer instead of yerler? Since the object is plural third person (onlar), wouldn't we use yerler instead? I thought yer was for singular third person (o).
As an addendum to what Alexin said, the -ler is only optional if we have mentioned the subject - çocuklar. If we leave that out and just say "they eat fruit," meyve yerler, it is necessary to include the -ler to indicate plurality.
Will the translation for "the children eat fruits" be "çocuklar meyveler yer" or "çocuklar meyveler yerler"??
"The children eat fruits" isn't really correct in English. It is really only used to distinguish different types (My three favorite fruits are...). It can also be a scientific term.
When you are just talking about eating fruit, it stays singular.
Anywho, you can never have the plural suffix on a general direct object. The translation of your sentence would still just be "Çocuklar meyve yer." :)